목재 기반 제품 산업에서의 나노기술 적용:검토

응용 프로그램

펄프 및 종이

전 세계적으로 종이와 판지의 연간 생산량은 4억 톤 이상입니다[22]. 현재의 디지털 시대가 지역 사회에서 종이 제품의 지속적인 사용을 멈추지 않았음을 보여줍니다. 그럼에도 불구하고 증가하는 수요는 대부분 다양한 포장 제품에 대한 것입니다[23]. 이러한 증가는 온라인 쇼핑에 대한 소비자 선호도의 변화와 집에서 편안하게 쇼핑할 수 있는 편리함을 제공하고 원하는 수령인에게 제품을 안전하게 배송할 수 있는 전자 상거래의 활성화에 박차를 가할 수 있습니다.

종이는 일련의 1차 가공, 처리, 제지, 건조 및 코팅을 거친 목질계 재료로 만들어집니다. 제지 단계는 펄프와 첨가제의 모든 혼합이 추가되고 다양한 최종 제품에 맞게 조정되는 단계입니다. 나노기술은 나노물질이나 나노첨가제의 형태로 펄프와 종이에 적용된다. 예를 들어, 산림 자원 또는 목질 섬유소 재료는 나노 크기의 빌딩 블록을 추출하거나 고유한 기계적 강도, 기능 및 유연성을 제공하기 위한 보강 단위 역할을 하는 나노셀룰로오스로 알려져 있습니다[5]. 이 새로운 세대의 셀룰로오스는 세포벽 박리를 통해 나노피브릴을 얻거나 나노 규모에서 결정질 셀룰로오스를 추출하여 생산됩니다. 나노셀룰로오스는 주로 종이 강도를 높이기 위해 제지 공정에 첨가됩니다. 종이 생산 과정에서 사용되는 다른 나노 물질로는 나노실리카, 나노제올라이트 등이 있다[22]. 제지 산업에 나노물질 또는 나노첨가제를 통합하면 제지 제품의 성능을 향상시키는 데 도움이 될 수 있습니다.

펄프 및 제지 산업에서 사용되거나 잠재적으로 사용될 수 있는 주요 첨가제는 다음 섹션에서 설명합니다.

습식 또는 건식 강화제로서의 나노셀룰로오스

나노셀룰로오스는 펄프 및 종이 분야에서 강도 첨가제로 광범위하게 연구되었습니다[24, 25]. 나노셀룰로오스는 풍부하고 유용하며 생분해성, 낮은 독성, 우수한 기계적 및 광학적 특성, 높은 표면적 및 재생성과 같은 흥미로운 특성 때문에 엄청난 관심을 받고 있습니다[5, 7]. 나노셀룰로오스는 실제로 목재 펄프, 비목재 식물, 목재 및 농업 잔류물, 세균성 셀룰로오스 및 튜니케이트를 포함하는 모든 리그노셀룰로오스 물질로부터 분리 및 제조할 수 있는 나노 규모의 셀룰로오스입니다. 셀룰로오스는 결정질 및 무정형 영역을 포함하는 β-1,4 연결된 포도당 단위를 가진 선형 화합물입니다. 무기산을 사용하여 셀룰로오스 사슬에서 무정형 도메인을 제거하면 나노결정질 셀룰로오스가 분리되는 반면, 고전단 기계적 작용을 통한 세포벽 박리는 셀룰로오스 폭을 감소시켜 나노피브릴화 셀룰로오스를 형성합니다. 나노셀룰로오스의 이 두 가지 범주는 제품 향상을 위해 펄프 및 종이 분야에서 가장 많이 연구된 나노물질입니다. 지난 10년 동안 다양한 식물 자원에서 나노피브릴화 셀룰로오스(NFC) 및 나노결정질 셀룰로오스(NCC)를 제조하는 데 많은 연구가 집중되었습니다. 그림 1은 투과전자현미경(TEM)을 사용한 나노결정질 셀룰로오스 및 나노피브릴화 셀룰로오스의 이미지를 보여줍니다.

Acacia magium의 NCC TEM 이미지 아 출처 :Jasmani와 Adnan [26]. 침엽수 펄프, b에서 Elsevier 및 NFC의 허가를 받아 복제 출처 :Zhao et al. [27]. Elsevier의 허가를 받아 복제

나노셀룰로오스의 특성은 제조 방법과 셀룰로오스의 공급원에 따라 다릅니다. 표 1은 나노피브릴화 셀룰로오스와 나노결정질 셀룰로오스의 형태와 결정성을 나타낸다.

셀룰로오스에서 나노셀룰로오스로

Nanofibrillated cellulose는 많은 연구가 보고된 것처럼 제지 분야에서 가장 많이 연구된 나노첨가제이다[3, 25]. Nanofibrillated 셀룰로오스는 섬유에 선택적 고전단 기계적 처리를 적용하여 제조됩니다. 나노피브릴화 셀룰로오스를 생산하기 위한 많은 방법과 방법의 조합이 있습니다. 균질화와 미세유동화는 나노피브릴화 셀룰로오스 제조에 사용되는 일반적인 기술로, 일반적으로 10~15회 반복되는 통과를 위해 고압 하에 섬유를 작은 노즐에 넣는 것을 포함합니다[32, 33]. 이러한 유형의 처리는 단독으로 사용할 경우 높은 에너지 소비가 필요하므로 TEMPO(2,2,6,6-tetramethylpiperidin-1-oxyl) 매개 산화[34], 카르복시메틸화 또는 효소[35, 36]와 같은 화학 물질로 전처리해야 합니다. ]는 일반적으로 에너지를 절약하는 데 필요합니다. 이 방법을 사용하여 나노피브릴화된 셀룰로오스는 유체 흐름에서 높은 속도와 힘에 의해 유도된 높은 전단 속도의 결과로 생성됩니다. 나노피브릴화 셀룰로오스를 생산하는 또 다른 대중적인 방법은 그라인딩 방법입니다. 연삭기는 정적 및 회전 석재 디스크로 구성됩니다. 돌에 의해 생성된 전단력은 세포벽 섬유를 개별화된 나노섬유로 분해합니다. 이 방법을 사용하여 응집으로 인한 폭의 넓은 분포를 피하기 위해 저자는 펄프화 및 표백 공정 후에 공급원료를 물에 보관할 필요가 있다고 권장했습니다. 이는 건조한 상태에서 수소결합 네트워크 형성을 방해하기 위함이다[37, 38]. 초음파 처리 방법은 나노 섬유를 준비하기 위해 연구되었습니다. 초음파 처리는 섬유 셀 구조를 파괴할 수 있는 초음파에서 캐비테이션 고전단력의 생성을 포함합니다[39]. 초음파 처리 기술은 잘 분산되고 안정적인 나노섬유를 생성한다고 합니다[3]. 그 외에도 저온 분쇄를 적용하여 천연 섬유를 나노 섬유로 변환할 수도 있습니다. 섬유는 액체 질소와 높은 전단력을 사용하여 동결해야 합니다. 높은 전단력은 얼음 결정에 압력을 가하여 세포벽이 파열되고 미세섬유를 방출하도록 합니다[40,41,42,43].

천연 셀룰로오스에서 추출한 나노결정질 셀룰로오스는 일반적으로 산 가수분해를 사용하여 제조됩니다. TEMPO 매개 산화[44] 및 이온성 액체[45]와 같은 특정 화학 물질의 사용을 포함하는 다른 방법도 나노셀룰로오스 제조를 위해 연구되었습니다. 셀룰라아제 효소의 적용도 보고되었다[46, 47].

산 가수분해 동안 셀룰로오스의 중합도는 급격히 감소하지만 중합도 수준으로 알려진 특정 지점에서 안정화됩니다[48]. 이러한 거동의 이유는 비정질 영역이 산에 의해 빠르게 가수분해되기 때문일 수 있습니다. 가수분해가 시작되면 산은 높은 부피로 인해 비정질 영역을 공격하고 쉽게 접근할 수 있는 글리코시드 결합을 가수분해합니다[49]. 쉽게 접근할 수 있는 결합을 가수분해하면 포도당 사슬의 환원 말단과 결정질 영역 표면에서 훨씬 더 느린 속도로 추가 가수분해가 발생합니다[50]. 산 가수분해는 산의 종류, 산 농도, 온도 및 시간과 같은 요인의 영향을 받습니다[49, 51, 52]. 산 농도, 가수분해 시간 및 온도 조건의 변화는 나노결정질 셀룰로오스의 형태에 영향을 미칩니다. 예를 들어, 가수분해 시간[53]과 산 농도[29]를 모두 증가시키면 나노결정질 셀룰로오스가 짧아지는 반면 고온에서는 셀룰로오스가 포도당으로 완전히 전환됩니다[50, 54, 55].

제지에서 나노셀룰로오스를 추가하면 종이의 강도[56, 57]와 밀도[58,59,60]를 증가시키고 다공성[61]도 감소시키는 더 나은 성능의 종이를 얻을 수 있습니다. 나노구조의 셀룰로오스를 추가하면 고운 섬유와 유사한 종이 특성이 추가됩니다[3, 25]. 이론적으로 종이의 강도는 습윤 및 건식 강화제의 포함[24, 62], 기능화된 섬유의 추가[63] 및 고해[64]에 의해 증가될 수 있습니다. 강화는 섬유 결합 능력을 가져온다[25]. Boufi et al. [25]는 이 메커니즘이 섬유 간 결합을 유도하는 섬유 사이의 연결자 역할을 하는 나노셀룰로오스의 결과로 결합 면적이 증가하기 때문일 수 있다고 제안했습니다. 그 외에도 광섬유에서 생성된 서로 다른 네트워크로 인해 결합 기능이 증가할 수도 있습니다. 나노셀룰로오스의 마이크로미터 길이가 주어지면 그들은 더 강한 네트워크로 이어지는 인접 섬유를 연결하는 다리 역할을 할 수 있습니다[25]. 섬유와 나노셀룰로오스에 의한 상호 결합 네트워크는 종이의 강도를 증가시킵니다.

나노 셀룰로오스는 나노 첨가제로 내부 결합을 개선하여 건조 인장 강도를 증가시키고 공기 투과도와 불투명도를 감소시키며 밀도를 높입니다[3]. 나노셀룰로오스는 나노크기의 물질이기 때문에 높은 표면적을 가지므로 보다 효과적으로 수소결합을 형성할 수 있다. 건조강도 향상 및 유지력 향상을 위한 Wet End 첨가제로 첨가됩니다. 일반적으로 건조강도 첨가제로 사용되는 나노셀룰로오스는 나노섬유화 셀룰로오스의 형태입니다.

NFC는 [24, 62, 65] 또는 보유 보조제 없이 직접 추가되거나 [58] 다른 충전제 또는 장섬유 [66, 67]와 혼합된 후 보유 보조제와 함께 혼합될 수 있기 때문에 펄프 퍼니쉬에 NFC를 추가하는 다양한 전략이 있습니다. . NFC 첨가 후 인장강도 증가는 첨가량에 비례한다. 고해된 펄프와 양이온성 전분으로 구성된 펄프 지료에 NFC 3%를 첨가하면 인장강도가 5% 증가하는 것으로 나타났다[24]. 펄프를 덜 치면 효과가 더 두드러진다는 점은 흥미롭습니다. 예를 들어, 열기계 펄프에 NFC 6%를 첨가하면 인장 강도가 100% 이상 증가합니다. 반면 NFC는 잘 섞인 화학펄프에 첨가했을 때 영향이 적었다[65]. 따라서 충전지, 기계펄프, 재생펄프에 NFC를 첨가하면 인장력 향상이 기대된다. Hii et al. [65]는 NFC가 필러와 섬유에 흡착되어 필러를 파이버 네트워크와 연결한다는 관찰을 보고했습니다. 제지에서 NFC를 사용할 때의 유일한 주요 단점은 배수가 느려진다는 것입니다. 배수는 종이 생산의 효율성과 직접적인 관련이 있기 때문에 제지에서 매우 중요한 역할을 합니다. 배수 시간이 느릴수록 용지가 더 느리게 생성됩니다. 따라서 섬유 표면에서 나노섬유의 흡착을 개선하여 탈수를 개선하기 위해 특정 용량으로 보유제를 사용하는 것이 매우 중요합니다[24, 68].

NFC는 전분과 같은 건조강도첨가제와 두드리는 작용에 의해 생성되는 미분의 중간 성질을 갖고 있어 둘 다 접착면적을 증가시킨다[3]. 이는 건조 중 섬유 결합을 증가시키는 데 도움이 되는 섬유 표면에 부드럽고 얇은 층을 생성하고 결합 면적을 증가시키는 섬유 사이의 공극과 기공을 채워서 달성됩니다[3].

배리어성 코팅 재료로서의 나노셀룰로오스

나노셀룰로오스는 차단성이 우수하여 포장지의 코팅재로도 사용할 수 있습니다. 코팅 요소로 나노셀룰로오스를 사용하는 이점은 종이를 만든 후에 추가되기 때문에 탈수 문제가 더 이상 문제가 되지 않는다는 것입니다. 스프레이, 바 코팅, 크기 압착 및 롤 코팅을 포함하여 나노셀룰로오스를 적용하기 위해 사용할 수 있는 다양한 접근 방식이 있습니다. 나노셀룰로오스, 특히 나노피브릴화 셀룰로오스의 적용은 산소 장벽과 내유성을 증가시키는 것으로 보고되었다[69]. 한 예에서 공기 투과도의 감소는 69,000에서 4.8로, 660에서 0.2nm Pa ⁻¹ 로 관찰되었습니다. 로드 코터를 사용하여 각각 [70] 표백지 및 그리스 방지지. Syverud와 Stenius[71]는 침엽수 펄프에 0%에서 8%까지 다양한 양의 NFC를 적용했으며 차단 특성이 6.5nm에서 360nm Pa ^-1 로 현저히 증가했음을 발견했습니다. s ⁻¹ . 이는 나노피브릴의 증가로 인한 다공성 감소 때문입니다. NFC와 셸락의 적용도 Hult 등에 의해 시도되었다. [72] 종이 및 판지에서 공기 투과성, 산소 투과율 및 수증기 투과성을 감소시켜 장벽 포장의 가능성이 있습니다.

뿐만 아니라, 나노셀룰로오스, 특히 나노피브릴화 셀룰로오스는 자립형 박막 또는 나노페이퍼로 전환될 수 있다. 이 나노종이는 투명하고 유연하기 때문에 전자적 응용을 위한 기판으로 사용될 수 있다[73].

재질 개선을 위한 보유제로서의 나노물질

유지제는 종이에 기능성 화학물질의 유지력을 향상시키기 위해 제지 공정에 첨가됩니다. 일부 나노물질은 나노제올라이트[74] 및 나노티타늄 이산화물의 사용과 같은 종이 제품에서 테스트되었습니다. 나노제올라이트는 제지 산업에서 수분을 흡수하는 건조제로 사용되며 특수 용지에 사용되는 경우 가스 배출을 제거하는 기능도 있습니다. 공극과 기공으로 구성된 나노제올라이트의 높은 표면적은 이러한 과정을 돕습니다. 종이에 첨가된 나노티타늄 산화물은 대조 샘플에 비해 더 나은 동적 탄성 계수를 갖는 종이를 형성할 수 있습니다[75].

속성 향상을 위한 나노 필러 효과

제지 산업에서 필러를 사용하는 이유는 필러가 일반적으로 펄프 자체보다 저렴하기 때문입니다. 강도 첨가제로 연구되는 것 외에도 나노셀룰로오스는 충전제로도 사용할 수 있습니다. 나노피브릴화 셀룰로오스의 첨가는 목재펄프의 양을 줄이고 충전재의 양을 증가시켜 생산원가를 감소시킬 수 있다[76]. 또한, 생산된 종이는 낮은 다공성 및 높은 불투명도와 같은 향상된 특성을 가지고 있습니다. 또한 2-10% 나노피브릴화 셀룰로오스를 충전제로 추가하면 강도가 50-90% 증가한다고 보고되었습니다(Future Markets Inc. 2012). 나노클레이는 종이의 저장 수명을 연장할 수 있는 가스 투과성을 줄이기 위해 제지에서 첨가제로 사용할 수 있습니다. 이는 가스 및 물 장벽이 식품 및 음료의 부패를 방지하는 데 중요한 역할을 하는 포장 산업에서 필수적입니다.

나노칼슘 카보네이트는 광산란을 개선하기 위해 필러로 사용됩니다. 나노구조 입자를 사용한 변형된 침강성 탄산칼슘은 광산란에 긍정적인 영향을 미쳤다[77]. 침전된 탄산칼슘을 규산염과 황화아연 나노입자로 코팅하였다. Wildet al. [78]은 나노입자 코팅이 실험실, 파일럿 및 공장 시험에서 사용된 유사한 연구를 보고했습니다. 이 연구는 나노 입자 코팅이 우수한 인쇄 품질, 내수성 및 치수 안정성을 제공한다는 것을 발견했습니다. 종이 가구에 첨가된 나노산화아연은 ​​종이에 항균성을 부여합니다. 동시에 나노산화아연을 첨가하면 종이의 인쇄성 뿐만 아니라 밝기, 백색도 등의 광학적 특성도 향상되었다. 나노티타늄 옥사이드는 코팅된 종이에서 베타-사이클로덱스트린과 조합하여 연구되었습니다[79]. 나노물질 혼합물은 나노티타늄 산화물만 코팅한 종이에 비해 크실렌에 대한 분해 효과가 더 우수함을 발견했습니다.

재질 개선을 위한 사이징제로서의 나노물질

사이징제(sizing agent)는 종이가 인쇄 및 필기용에 적합하도록 물/액체 침투에 대한 저항성을 향상시키기 위해 제지 상에 첨가된다. 나노실리카의 사용은 광학적 특성을 향상시키고 프린트스루를 최대 30%까지 감소시킬 수 있습니다. 나노실리카로 코팅된 종이는 코팅되지 않은 종이보다 더 나은 광학 밀도, 치수 안정성, 인쇄 품질을 나타내는 것으로 밝혀졌습니다[80,81,82].

목재 합성물

목재는 다양한 용도로 사용할 수 있는 생분해성 및 재생 가능 소재이기 때문에 자연이 인류에게 준 선물입니다. 그러나 나무 자체는 흰개미의 공격 등으로 인해 섬세하고 유연하지 않으며 내구성이 떨어지는 등 여러 가지 약점을 가지고 있습니다. 목재 복합 재료를 생산하는 데 목재 섬유를 사용하는 것은 낮은 부피 밀도, 낮은 열 안정성, 높은 수분 흡수 경향 및 생물학적 분해에 대한 민감성을 갖기 때문에 자체적인 단점이 있습니다. 나노기술은 많은 과학 분야에서 활용되어 왔으며 목재 및 목재 복합 재료를 비롯한 많은 재료의 품질을 개선하는 데 사용될 수 있습니다.

보강재로서의 나노셀룰로오스

나노셀룰로오스를 매트릭스 재료의 보강재로 사용하는 원리로 인해 많은 연구가 진행되고 있다. 나노크기의 셀룰로오스를 첨가함으로써, 나노복합체는 마이크로복합체로는 달성할 수 없는 다양한 방식으로 뛰어난 특성을 갖는다[83]. 이러한 나노셀룰로오스 강화 복합재는 기존 복합재를 대체할 수 있습니다. NCC의 적절한 변형을 통해 물리적, 화학적, 생물학적 및 전자적 특성이 크게 개선되거나 우수한 특성을 가진 다양한 기능성 나노물질을 개발할 수 있습니다. 나노복합체의 특성은 매트릭스 물질의 특성, 나노셀룰로오스의 특성, 매트릭스 물질에서 나노셀룰로오스의 분산, 충전제와 매트릭스 물질 사이의 계면 상호작용과 같은 몇 가지 요인에 의존합니다[84].

다양한 유형의 매트릭스 재료에 대한 보강재로서의 나노셀룰로오스가 널리 연구되었습니다. 나노셀룰로오스로 강화된 천연 또는 합성 고분자로 구성된 고분자 나노복합체는 유망한 종류의 재료로 자리 잡았습니다. 기계적 특성의 개선은 이러한 나노셀룰로오스 강화 폴리머를 제조할 때 목표로 하는 가장 일반적인 목표입니다[85,86,87]. 석유계 고분자는 일반적으로 열가소성과 열경화성으로 나뉜다. 열가소성 중합체와 열경화성 중합체의 차이점은 장쇄 분자를 유지하는 결합이며, 전자는 약한 반 데르 발스 결합에 의해 유지되고 후자는 강한 공유 결합에 의해 유지됩니다[88]. 다양한 열경화성 수지가 나노셀룰로오스 복합 재료에 사용하기 위해 연구되었습니다. 예를 들어, 에폭시 수지는 우수한 접착 특성과 경화 후 우수한 기계적 특성(높은 모듈러스, 낮은 크리프 및 합리적인 고온 성능)으로 인해 고급 재료 제품에 사용되었습니다. 그러나 고도로 가교된 구조로 인해 충격을 받으면 쉽게 파손될 수 있습니다[89]. 강화 물질로 기능화된 나노셀룰로오스를 추가함으로써, 에폭시 기반 셀룰로오스 나노복합체의 기계적 특성이 크게 향상되었습니다[90, 91]. 방향성 구조를 갖는 에폭시-셀룰로오스 나노섬유 복합재료를 제조하기 위한 접근이 수행되었다[92]. 이 공정은 바이오에폭시 수지 함침을 위한 프리폼으로 사용되기 전에 고다공성 나노셀룰로오스 네트워크를 준비하기 위해 얼음 템플릿(또는 동결 주조) 방법을 결합했습니다. 그 결과, 나노복합체의 탄성 및 저장 탄성률은 두 시험 방향 모두에서 순수 에폭시보다 더 우수하고 길이 방향으로 강도가 향상됨을 보여주었다. 또 다른 가장 일반적으로 사용되는 열경화성 수지는 불포화 폴리에스테르(UP)입니다. 이전 연구보다 훨씬 높은 45vol%의 높은 나노셀룰로오스 함량을 가진 나노구조의 UP 생체복합체는 성공적으로 처리되고 특성화되었습니다[93]. 나노구조의 나노셀룰로오스 네트워크 보강재는 UP의 모듈러스와 강도뿐만 아니라 연성 및 인성도 크게 향상시킨다. 그림 2는 전계 방출 주사 전자 현미경(FE- SEM). FE-SEM 이미지는 아세틸화된 NCC가 NCC와 비교하여 PHB에 균질하게 분산되었음을 분명히 나타내었다. 균질성은 보강재와 폴리머 매트릭스 사이의 강력한 계면 상호작용에 기여했습니다[94].

a의 골절 형태에 대한 FE-SEM 이미지 PHB/NCC-15, b PHB/아세틸화 NCC(II)-15, c PHB/아세틸화 NCC(IV)-15 나노복합체. 출처 :Gan et al. [94]. ACS의 허가를 받아 복제

표 2는 PLA, PVA, 전분, PU, ​​PP 등과 같은 열가소성 폴리머에 나노셀룰로오스의 통합을 보여줍니다. 이러한 나노복합체를 개발하는 데 사용되는 공정에는 용매 교환, 수성 분산액, 용액 주조, 그래프팅, 코어백 발포체 사출 성형, 전기 방사, 응고 및 열압착, 제자리 음이온 개환 중합 반응 등이 있습니다. 대부분의 연구에 따르면 나노복합체에 나노셀룰로오스를 첨가하면 구성요소에 따라 기계적 특성(강도, 강성, 내크리프성, 탄성), 열안정성, 차단성, 심지어 일부는 나노복합체의 항균 및 항산화 기능이 향상된다고 보고했습니다. 사용된 나노복합체. 전반적인 결과는 더 친환경적이고 지속 가능한 복합 재료 생산에 대한 더 많은 연구를 촉진하고 장려하는 열가소성 매트릭스의 나노셀룰로오스 강화에 대한 긍정적인 결과를 보고했습니다.

천연 고분자는 일반적으로 자연에서 발생하며 추출할 수 있습니다. 천연 폴리머는 몇 가지 예외를 제외하고는 열가소성이 아닙니다. 그러나 화학적 및 물리적 변형 기술은 셀룰로오스, 리그닌 및 키틴과 같은 바이오매스 자원에서 천연 폴리머의 열가소성을 유발할 수 있습니다. 환경에 대한 인식이 높아지고 친환경 제품에 대한 수요가 높아짐에 따라 나노셀룰로오스를 강화한 다양한 천연고분자가 바이오나노복합체 생산에 활용되고 있다. 뼈 조직 재생에 식물 유래 나노셀룰로오스의 사용을 보고하는 연구는 매우 제한적입니다. 연구 중 하나[113]는 수산화인회석(HA)으로 (TEMPO) 산화된 나노피브릴화 셀룰로오스(TNFC) 또는 나노결정질 셀룰로오스(NCC)를 합성하여 나노복합체를 제조한다고 보고했습니다. 복합 재료는 외부 및 조밀한 피질골 범위에서 NCC 기반 복합 재료보다 더 나은 압축 강도, 탄성 계수 및 파괴 인성을 나타내는 것으로 밝혀졌습니다. 또한, 복합 재료는 인간의 뼈 유래 조골 세포에 세포 독성을 유도하지 않고 오히려 생존력을 향상시켜 하중을 받는 응용 분야에서 뼈 조직 재생을 약속했습니다.

사용되는 다른 천연 생체 고분자 중에는 생분해성, 생체 적합성 및 낮은 독성과 같은 특성을 갖는 알긴산나트륨, 셀룰로오스 및 단백질이 있습니다. 예를 들어, 알긴산 나트륨은 조직 공학, 약물 전달, 식품 포장 및 생물 의학 응용과 같은 많은 분야에서 우수한 생체 재료로 널리 사용되었습니다. 그러나 기계적 강도가 낮고 열화 특성이 제어되지 않아 적용이 제한됩니다. 알지네이트 매트릭스에 나노셀룰로오스를 혼입하여 나노복합체 필름을 개발함으로써 이러한 문제를 극복하기 위한 여러 시도가 수행되었다. NFC를 알지네이트 매트릭스에 통합하면 내수성과 기계적 특성이 향상되는 것으로 나타났습니다. 추가 조사에 따르면 분산을 촉진하기 위한 초음파 처리로 TEMPO 매개 산화 NCC는 NFC에 비해 알지네이트 생체 고분자를 강화하는 데 더 나은 효율성을 나타냅니다. 나노셀룰로오스를 키틴, 키토산, 분리 대두 단백질(SPI) 및 아마씨 검 매트릭스에 통합하는 생체 영감 시너지 강화 전략의 유사한 개념은 고성능 나노복합체를 구성하기 위한 새로운 길을 열어줍니다.

목재 패널 특성 향상을 위한 나노입자

목재 복합 재료는 일반적으로 바인더에 의해 결합된 목재 요소의 조합을 갖는 광범위한 제품으로 설명됩니다. 목재 복합 재료의 장점 중 하나는 두께, 등급 및 크기가 다른 특정 품질 또는 성능 요구 사항에 맞게 설계할 수 있다는 것입니다. 목재 합성물은 목재의 자연적인 강도 특성을 이용하도록 제조됩니다(때로는 일반 목재보다 구조적 강도와 안정성이 더 큼). On the other hand, wood composites also have disadvantages such that they require more primary energy to manufacture when compared to solid lumber. Hence, wood composites are not suitable for outdoor use as they can absorb water and are more prone to humidity-induced warping than solid woods. The adhesives used release toxic formaldehyde in the finished product. Nanotechnology can be utilised to improve the quality of wood-based composites to fulfil the increasing demand for existing products and for new products to be used in new applications.

The main drawbacks of wood are its susceptibility and biodegradability by microorganisms and also dimensional instability when subjected to varied moisture content. These are mostly due to the cell wall main polymers and their high abundance of hydroxyl groups (OH) [114]. Wood is naturally hygroscopic, and moisture absorption by wood materials is directly related to the exposed surface area. The addition of inorganic nanoparticles to wood composites has been reported to enhance the composites’ anti-microbial properties. Nanoparticles of zinc oxide (ZnO) exhibit good antimicrobial activity. These nanoparticles are added into melamine-urea formaldehyde (MUF) glue before being used for particleboard production [115]. The findings show that there were increments in bioresistance of the particleboards against the Gram-positive bacterium Staphylococcus aureus , the Gram-negative bacterium Escherichia coli , the molds Aspergillus niger and Penicillium brevicompactum as well as the brown-rot fungus Coniophora puteana . Silver nanoparticles which are well-known biocide additives also exhibited similar antibacterial and anti-mold efficiency effects when applied onto the melamine-laminated surfaces of particleboards [116]. The combination of nanocopper oxide and alkane surfactant was also confirmed to improve water and termite resistances of treated plywood specimens [117]. Modified starch-based adhesive was explored as another option to increase the decay resistance of particleboard. Particleboard bonded with modified PVA/oil palm starch added with nanosilicon oxide (SiO₂ ) and boric acid was found to be more decay resistant than particleboard bonded with their native starch [118]. The addition of nano-SiO₂ and boric acid as the water repellent and antifungal agents, respectively, have prevented the microorganism's activity in the final particleboard.

The manufacture of wood composite panels can be improvised by developing methods to shorten the cure time of the resin during hot-pressing, which could speed up production or improve overall quality of the board. Heat transfer which effects the pressing time of a wood-composite panel varies with thickness, press temperature, closing rate, and mat moisture distribution. The addition of ZnO nanoparticles increased the heat transfer at the centre of the particleboard during hot-pressing causing a greater degree of resin cure and improved the physico-mechanical properties [119]. High conductive nanoparticles such as multiwalled carbon nanotubes (CNTS) and aluminium oxide (Al₂ O₃ ) were also proven to enhance thermal and mechanical properties of medium density fibreboard [120]. The study also reported that although activated carbon nanoparticles did not give any significant effect to physical and mechanical properties of the board, they have more accelerated effect on the curing of urea formaldehyde (UF) and reduction in the formaldehyde emission compared to the other two nanofillers.

In fabricating the wood composites, the adhesives play exceptionally significant role which affect the composites properties which include the mechanical properties, their ability to perform in wet conditions and their effects on the environment. Urea formaldehyde, melamine urea formaldehyde and phenol formaldehyde are commonly used in the wood composites industry. The utilisation of nanoparticles has led to improvements of the properties of adhesives. Many studies have been conducted to produce nanomaterial-reinforced wood composites with enhanced physical and mechanical performance and reduced formaldehyde emission. Nanoclays have been shown to be excellent fillers and reinforcement for the resin matrix, and significantly enhancing strength, toughness and other properties. The modification of UF adhesive using nanoclay particles for plywood fabricated with three forest species from fast-growth plantations:Cordia alliodora , Gmelina arborea and Vochysia ferruginea were reported by Muñoz and Moya [121]. It was determined that nanomodification of the resin with nanoclay at 0.75% improved the moduli of rupture and elasticity of the board. The effect of using nanoclay particles in PF adhesives significantly elevated mechanical properties of the adhesive in the bondline and contributed an increase in the macro-bonding strength of plywood [122]. Interestingly, transition metal ion-modified bentonite (TMI-BNT) nanoclay was used to covert crystalline UF resins to amorphous polymers by blocking the hydrogen bonds via in situ intercalation method [123]. This resulted in 56.4% increase in the adhesion strength and 48.3% reduction in the formaldehyde emission.

Other nanoparticles used to improvise the physical and mechanical properties of wood composites include nanowollastonite (NW) [124, 125] nano-ZnO [126], nano-SiO₂ , nano-Al₂ O₃ [127], nanosilver [128] and nanocellulose (NCC and NFC). The NCC was utilised as filler for the adhesive, whereas NFC was applied as a binder to the formulation of the composite boards. Recently, it has been revealed that the addition of micro- and nanofibres of cellulose have advantageous effects on the properties of resin. Based on the investigations, it was found that the addition NCC significantly improved the mechanical properties of plywood in the amount of 10%/100 g of solid resin [129]. On the other hand, the physical properties of particleboard after adding the NCC to UF adhesives showed smooth surface, insignificant difference in the density and moisture content of the panels and only high value of nanocellulose content exhibited significantly higher in thickness swelling [130]. Meanwhile, particle boards panels manufactured using NFC as the bonding materials were shown to meet the industry requirements in terms of mechanical properties for low density grades [131]. For high density particleboard, it was estimated that the increased NFC ratio and higher pressure could improve internal bond properties. Even the nail and face screw withdrawal strength was found to be increased with the increase in NFC addition ratio and panel density [132, 133].

Wood Coating

Forests are primarily or partially used for the production of wood and non-wood forest products. Non-wood forest products include bamboo, rattan, firewood, charcoal, damar, palm, etc. There is a huge demand for high quality wood, but the availability of wood from natural forest has been declining. Consequently, the search for non-wood resources as an alternative to wood has been accelerated. Due to its rapid growth property, bamboo has been developed into one of the most important non-wood forest products. Wood is a natural biologically self-assembled polymeric structure (cellulose, lignin, hemicellulose). It is one of the most versatile materials and has been used for centuries in the form of building and structures. However, wood is subjected to intense oxidative degradation processes such as photo-oxidation, chemical oxidation, thermal decomposition and photolysis reactions from the environment, including ultraviolet (UV) light, moisture, chemical pollutant and heat/cold variations [134]. Even non-wood materials like bamboo itself is a natural organic material which is rich in protein, carbohydrate and other nutrients and is prone to mildew, being eaten by moths and rotting. Hence, the final products of wood and non-wood products conventionally comprise additives which can be used as coatings for protection and aesthetic appearance improvement, preservatives for protection against fire and biological factors (fungi and insects).

Coating is a process of applying a layer to the substrate surface. Examples of common coatings applied to wood surfaces are varnishes, lacquers, and paints whose purpose can be both protective and decorative. The main components of the coatings determine their fundamental properties such as binders, pigments, solvents, fillers, and additives [135]. Each element contributes specific properties to the wood surface. The binder contributes to the adhesion of the pigment to the wood and creates a protective layer, while the pigment provides colour and form non-translucent surface layer. The solvents give necessary viscosity for coating application, and the addition of the fillers alters the colour strength and the gloss of the coating. As for the additives, they inhibit mould and decay, assist the drying process, improve the adhesion properties and control the finishing. However, there are weaknesses of coatings such as limited flexibility, strength loss, disproportionate adhesion between coating layer and substrate, inferior abrasion resistance and less durability.

Nanocoating has the capability to resolve these issues. Nanocoating is a process by which a thin layer of thickness about < 100 nm is deposited on the substrate for improving some properties or for imparting new functionality. The nanocoating can be used not only on nanomaterials but also on a bulkier material with an extremely thin layer coating without affecting the topography of the substrate surface. The application of nanocoating in wood and wood products is mainly focused on the improvement of durability, mechanical properties, fire resistance and UV absorption as well as decrease in water absorption. One approach to enhance the functionality and the end user value of nanocoating is the addition of nanoparticles [136]. These nanoparticles have very large surface-to-volume ratios due to their morphology, which allows them to interact intensively with their surroundings, and their nanosize ensures transparency is still sustained.

Nanoadditive for Durability Improvement

Nanocoatings are able to improve the durability of wood and non-wood products by utilising nanoparticles and nanodelivery systems that make the changes at the molecular level of the products. One of the aims of coatings is to prevent the growth of various microorganisms like fungi and bacteria. Nanosized particles of metal oxides, such as zinc oxide (ZnO) [137, 138], titanium oxide [138, 139] and cerium oxide (CeO₂ ) [140] were reported to demonstrate strong antimicrobial properties. Studies have been conducted in the direct deposition of nanoparticles onto wood surfaces or direct functionalisation of wood surfaces with nanoparticles. ZnO nanoparticles were successfully fabricated on the surface of bamboo timber by a simple low-temperature wet chemical method based on sol–gel-prepared ZnO seed layers. The findings indicated that the treated bamboo timber had better resistance against Aspergillus niger V. Tiegh (A. niger) and Penicillium citrinum Thom (P. citrinum), but poor resistance against Trichoderma viride Pers. ex Fr (T. viride) [141]. Graphene also demonstrates superior ability to inhibit bacterial growth. Hence, the combination of utilising reduced graphene oxide and nano-ZnO to coat bamboo-based outdoor materials via a two-step dip-dry and hydrothermal process, resulting in the improvement of the mould resistance and antibacterial activity properties [142]. Similarly, the nanostructured ZnO using a hydrothermal process has also provided an effective protection of wood surfaces from biodeterioration [143].

Waterborne polyurethane [144] coatings (WPU) incorporated with nanocrystalline cellulose (NCC) and silver nanoparticles (AgNPs) were used to improve antibacterial property of wood board [145]. The AgNPs were known for their antimicrobial material but aggregated easily during the preparation process. Therefore, NCC was introduced to assist with the blending and dispersibility of AgNPs with WPU or other coatings. In addition, NCC was also a good reinforcing agent to improve the mechanical properties of nanocomposites. Mini emulsion polymerisation was also used to synthesise an acrylic latex coating containing AgNPs, which will limit the growth of black-stain fungi on the wood surfaces [146]. The study of the antibacterial effect of silver and zinc oxide nanoparticles in acrylic coatings applied during the treatment of commercial wooden composites such as particleboard and medium density fibreboard was conducted [147]. Ag and ZnO nanoparticles were partly more effective against the Gram-negative bacterium Escherichia coli compared to the Gram-positive bacterium Staphylococcus aureus .

Nanoadditive for Water Absorption Improvement

It is well known that wood is susceptible to water or moisture. This is due to the hydrophilic nature of the cell wall constituent polymer and its capillary-porous structure. The interaction between wood and water leads to biodegradation of wood, dimensional instability and accelerated weathering. Although there are conventional chemical modifications being used to improve the hydrophobicity of wood, the accessibility of water into wood is still not completely retarded [148]. Furthermore, the chemicals used in the treatment process are possibly hazardous. Hence, nanotechnology is used as an alternative approach for wood modification and functionalisation. The incorporation of nanoparticles into polymeric coatings is used to improve water absorption of wood surfaces. Two approaches are utilised to integrate the nanoparticles into the coatings, namely solution blending and in situ addition [149]. The first approach (solution blending) is when a solvent combines with the polymer before being dispersed onto the wood surfaces. This physical method can be applied through dipping, brushing and spraying [150]. Wu et al. [151] reported that a superhydrophobic coating was constructed on the surface of poplar wood with a contact angle of up to 158.4° through the waterborne UV lacquer product (WUV) which was modified by ZnO nanoparticles and stearic acid. Compared with WUV, the water resistance of zinc stearate/waterborne UV lacquer super-hydrophobic coating (ZnSt₂ /WUV) was stronger, which was conducive to prepare superhydrophobic coatings in an easy and environmentally friendly. Interestingly, a water-based varnish added with TiO₂ nanoparticles was used to evaluate the finishing of nine tropical wood species [152]. It was found that the incorporation of TiO₂ nanoparticles decreased the values of water absorption and after a year of weathering exposure, the varnish with no added TiO₂ nanoparticles degraded completely, while the modified varnish film endured. Other examples of superhydrophobic wood coatings which were successfully prepared are lignin-coated nanocrystalline cellulose (L-NCC) particles/polyvinyl alcohol (PVA) composite paint system [153], UV-light curable methacrylic-siloxane-cellulose composite coatings [154], Fe ³⁺ -doped SiO₂ /TiO₂ composite film [155] and polydimethylsiloxane (PDMS)/silica hybrid coating system [156].

The second approach is the in situ addition or a chemical process which involves compound addition directly to monomers and subsequent polymerisation. The nanoparticles are synthesised in situ by chemical reactions on the wood surface such as hydrothermal methods or solgel deposition. Gao et al. [157] applied a simple and effective method in preparing superhydrophobic conductive wood surface with super oil repellency using AgNPs modified by fluoroalkyl silane. The multifunctional coating could be commercialised for various applications, especially for self-cleaning and biomedical electronic devices. In another study, bamboo was treated using ZnO sol, and the ZnO nanosheet networks were grown hydrothermally onto the bamboo surface and subsequently modified with fluoroalkyl silane [158]. The successfully treated bamboo exhibited superior properties such as robust superhydrophobicity, stable repellency towards simulated acid rain, UV-resistant and fire-resistant. Similar superior properties were obtained when bamboo timber prepared by the hydrothermal deposition of anatase TiO₂ nanoparticles and further modified with octadecyltrichlorosilane [159]. Superhydrophobic wood surfaces can also be prepared using approaches such as layer-by-layer [160] assembly of polyelectrolyte/TiO₂ nanoparticles multi-layers and hydrophobic modified with perfluoroalkyltriethoxysilane (POTS) [161], spray coating of a waterborne perfluoroalkyl methacrylic copolymer (PMC)/TiO2 nanocomposites onto the PDMS pre-coated substrate [162] and a one-step hydrothermal process using tetrabutyltitanate (Ti(OC₄ H₉ )₄ , TBOT) and vinyltriethoxysilane (CH₂ CHSi(OC₂ H₅ )₃ , VTES) as a co-precursor [163]. Even a biomimetic approach as to produce a lotus-leaf-like SiO₂ superhydrophobic bamboo surface based on soft lithography was successfully carried out [122].

Nanoadditive for Mechanical Properties Improvement

The inorganic particles integrated into organic polymers are commonly used in wood coatings to increase the mechanical properties. As fillers, the rigidity and hardness of the inorganic materials are combined effectively with the polymer’s processability. The inorganic particles when apply in micron size have disadvantages such as they reduce the flexibility of the material and decrease the transparency of the coating system [164]. The utilisation of the inorganic particles in nanosize increases the surface area and the ratio of the interfacial area, which subsequently influences the properties of the raw material [165]. Recent studies have investigated on using nanocellulose as a renewable reinforcement to develop a bio-based nanocomposite coating system with improved performance. Nanocellulose was surface modified due to the issue of incompatibility with the polymer matrix. The addition of TEMPO-oxidised cellulose nanofibres improved the mechanical properties of the WPU coating [144, 166]. In the case of the non-polar polymer matrix [167], nanocrystalline cellulose was modified by two methods, with acryloyl chloride or a cationic surfactant [167]. An increase in NCC loading level up to 2% increased hardness, elastic modulus, and tensile strength.

Nanosilica is another common nanoparticle that is applied for the improvement of mechanical properties. Among the advantages of using nanosilica are its high hardness and can easily be chemically modified to improve its compatibility with the polymer matrix. A recent study by Meng et al. [168] reported castor-oil-based waterborne acrylate (CWA)/SiO₂ hybrid coatings with organic–inorganic covalent cross-linked network structures were prepared via solgel and thiol-ene reactions. The finding showed that beside the emulsions had good stability, the thermal and mechanical properties of the coating improved significantly at 10 wt% of SiO₂ . The improvement of mechanical properties with nanosilica addition was also described in other coating system such as waterborne nitrocellulose [169] and acrylate [170].

Nanoadditive for UV Absorption Improvement

The process of wood photodegradation begins directly after being exposed to solar light, and then the colour changes and progressive erosion of the wood surface occur. The UV radiation is capable of photochemically degrading the polymer structure components of wood (lignin, cellulose and hemicellulose) [171]. The photodegradation process usually results in reduced water resistance of wood and wood-based materials which lead to further biodegradation under outdoor exposure conditions. The intense damage to materials due to the UV component in solar radiation can be prevented by using light-stabilisation technologies, surface coatings or by replacing these materials with materials that are more resistant against UV radiation [172]. Nanoparticles can be utilised to improve the UV resistance for solvent, waterborne and UV coatings in order to protect the wood surfaces. Nanoparticles that contain functional coatings to achieve UV-blocking properties offer a high level of protection against UV without affecting the transparency of the surface. The small size of the nanoparticles gives a significant increase in effectiveness of blocking UV light compared to natural material due to their large surface area-to-volume ratio.

The use of UV radiation absorptive coatings serves to prevent lignin degradation from UV light. Among the nanoparticles used as UV absorbers are mainly TiO₂ and ZnO. Wallenhorst et al. [173] reported a system composed of a Zn/ZnO coating and additional polyurethane sealing strongly reduced photodiscolouration of the wood surface and proved to be chemically stable. The combination of benzotriazole (BTZ) and ZnO nanoparticles was applied as the UV absorbers in acrylic-based bamboo exterior coatings [174]. Strong synergistic effects were detected in the BTZ–ZnO coatings, especially for the 2:1 ratio formulation. The coating system provided high resistance to photodegradation and effectively inhibited photodiscolouration of the bamboo substrates. Another mixture of benzotriazoles, hindered amine light stabilizers (HALS), and ZnO nanoparticles in thick-film waterborne acrylic coating also gave the most positive effect in UV protective surface modification when applied to oak wood [175]. The mixture of benzotriazoles, HALS and both TiO₂ and ZnO nanoparticles was confirmed as one of the most effective treatments for colour stabilisation of wood due to UV and VIS spectrums. It was reported that wood specimens coated with rutile TiO₂ and a mixture of methyltrimethoxysilane and hexadecyltrimethoxysilane showed superior weathering performance and improved resistance to surface colour change and weight loss [176]. TiO₂ coating also was found to apparently enhance the colour stability of wood during UV light irradiation without water spray. However, the adjacent wood surface degraded because of the photocatalytic activity of TiO₂ [177].

Nanoadditive for Fire Retardancy Improvement

The flammability of wood and non-wood products has restricted utilisation, with fire safety being a major concern for the various applications. To overcome the inherent deficiencies and use of wood and non-wood in a safe manner, the flame retardant properties need serious consideration. Nanoparticles have recently been used to produce the nanocomposites for the improvement in fire retardant properties. The utilisation of nanoparticles, either alone or in combination with conventional fire retardants, serves to reduce the ignitability of wood. The nanosize and high surface area of nanomaterials make them more effective at low concentrations than other conventional compounds which are an enormous advantage industrially and economically. The surface modification is necessary for nanoparticles to achieve better compatible and homogeneous dispersion. TiO₂ coated wood was found to be capable in reducing the flammability of the wood and the spreading of the flame, as compared to the uncoated sample [178]. The ZnO–TiO₂ -layered double-nanostructures had been synthesised on a bamboo substrate [179]. The findings showed that the oxygen index increased from 25.6 to 30.2% after being covered with a ZnO–TiO₂ coating, which revealed a significant enhancement of its flame retardant property. Layered double hydroxides [180] can absorb a large amount of heat, dilute the concentration of flammable gas, and absorb harmful acid gases during the decomposition process; therefore, it is an excellent flame retardant. Yao et al. [180] applied nanomagnesium aluminium layered double hydroxide (Mg–Al LDH) to bamboo in an in situ one-step process and found that the total heat release and total smoke production were reduced by 33.3% and 88.9%, respectively, compared to those of samples without Mg–Al LDH. Wang et al. [181] introduced zinc-aluminum layered double hydroxide (Zn–Al LDH) nanostructures to wood and found that the peak heat release rate (PHRR) and total smoke production were reduced by 55% and 47%, respectively, compared to those of the pristine wood [181]. Nanostructured carbon materials such as graphene was also proven to have a great potential to be used as an effective fire retardant in wood and wood-composite materials for surface protection against fire [182].

Wood Durability

Wood is such a versatile material that finds its use in various fields like construction, furniture and artwork [183,184,185]. Wood is applied as a construction material due to its high strength-to-weight ratio, eco-friendly characteristic, aesthetic appearance and biodegradability feature. Unfortunately, wood is very sensitive to biological attacks, especially by decay fungi and insects [186, 187]. Wood also gets affected by exposure to UV-radiation, fire and moisture [188]. Moisture can cause wood warping, cracking and dimensional instability. The wood degradation can cause aesthetical and internal structural damages. The degradation of wood leads to immeasurable losses each year. Besides, the growth of decay fungi on wood structural will trigger health problem to human including allergies, respiratory symptoms and asthma especially after prolonged indoor exposures [189]. Therefore, the associated issues related to wood cannot be simply overlooked. Preservation of wood using chemical is the effective way to protect and prolong the service life of wood.

Over the last decades, many chemical preservatives have been developed and used to protect wood against biodegradation agents [188, 190]. Unfortunately, most of these chemical preservatives may pose serious effects to human, living organisms and environment due to their accumulation in soil and ecosystems [191]. Chromated copper arsenate (CCA) is one of the chemical preservatives widely used since the middle 1930s and effectively protects wood against decaying fungi, termite and insect borer. However, CCA was shown to be toxic to human and environment [192,193,194]. A similar issue is also faced by another chemical, i.e. pentachlorophenol (PCP). It was considered to be hazardous to human health; thus, its production and uses were banned in many countries [195].

In response to these issues, a new series of chemical preservatives claimed as safe and less-polluting are being introduced. These chemical preservatives can be obtained from plant extract or produced synthetically and have plant bioactive compound properties. For instance, pyrethroids, derived from pyrethrum flower (Chrysanthemum cinerariifolium ), have a potent insecticidal activity and can now be produced synthetically. Unfortunately, these chemicals were found easily degradable by light and temperature, and have narrow efficiency spectrum [196]. Other limitation of organic pesticides is that most of them dissolve in organic solvents and cannot be formulated as water-based wood preservatives. Therefore, smart and intelligent organic pesticides delivery system is crucial. Through a smart delivery system, biocides can be delivered in a controlled and targeted manner. This will reduce hazards to human and environment [17].

Wood treatment via nanotechnology method can improve wood durability against biodegradation agents and weather [14, 197, 198]. The main advantage of applying nanoproducts to wood is its unique ability to penetrate deeply in wood structure [14, 199], thus improving durability properties that result in a long life time service. On the other hand, complete penetration with uniform distribution of nanoproducts can also be achieved [200]. To date, many ready-to-use nanoproducts (nanopreservatives) for wood protection are available in the market, while some of them are still in the research and development stage. Generally, nanoproducts used to protect wood can be classified into two types, namely nanocapsules and nanomaterials. Nanocapsule refers to the pesticides embedded in a polymeric nanocarrier, while nanomaterial is nanosized metals which can be directly impregnated into wood [201].

Biocide Enhancement Property via Nanoencapsulation

Encapsulation of pesticides into polymeric nanocarriers is one of the promising nanotechnology techniques in improving the impregnation of wood with pesticides. This technique is designed to increase the solubility of poorly water-soluble pesticides and to release pesticides in a slow manner [202]. Encapsulation allows those low solubility pesticides to disperse easily in solid polymeric nanoparticles. The polymer can then be suspended in water and applied to wood via conventional water-based treatments [149]. Encapsulation is also able to protect the hydrophilic active ingredient from excessive leaching [19]. Due to the small diameter of capsule, it can be easily incorporated and penetrated into the cell wall of the wood. This accordingly improves the durability of treated wood against biodegradation agents. Not only this technology can deliver pesticides safely, it can also prolong the pesticide lifespan, resulting in extended protection for the wood [203].

Encapsulation of pesticides is a bottom-up approach. It can be carried out through several techniques such as nanoprecipitation [17, 204,205,206], emulsion-diffusion [207] and double emulsification [208] (Table 3). The materials used as polymeric nanocarriers can be derived from natural polymer, synthetic polymer or combination of both [209]. For instance, natural polymer includes cellulose, starch, alginates, silica and halloysite. The synthetic polymer usually used as polymeric nanocarriers includes poly(vinyl acetate), poly(methyl methacrylate), poly(lactic acid), etc. (Table 3).

A previous study by Liu et al. [18, 216] successfully encapsulated tebuconazole and chlorothalonil into polyvinylpyridine and polyvinylpyridine-co-styrene using the impregnation method. The particle size of capsules obtained was between 100 and 250 nm. They impregnated a suspension of the capsules into sapwood of southern yellow pine and birch using conventional pressure treatments. The treated wood was then exposed to brown-rot (Gloeophyllum trabeum ) and white-rot (Trametes versicolour ) fungi for 55 days. The results showed great resistance against both decay fungi.

Salma et al. [17] used the nanoprecipitation technique to encapsulate tebuconazole. The polymer capsules containing tebuconazole were prepared from amphiphilic copolymers of gelatine grafted with methyl methacrylate with the size diameters ranging from 200 to 400 nm or 10 to 100 nm depending on core/polymer shell mass ratio. The encapsulated tebuconazole was reported to be able to preserve wood against a brown rot fungus. On the other hand, the formulation system developed by Salma et al. [17] is flexible and can be easily modified using copolymerisation of other acrylic monomers like hydroxyethyl methacrylate. This indicates that tebuconazole release rate can be tailor-made. However, the disadvantage of this nanocapsules is that they are prone to aggregation, which reduces the delivery efficiency of the nanocapsules into the wood.

Can et al. [217] successfully encapsulated nano silver into polystyrene-soybean co-polymer. In their study, the Scots pine was impregnated with the capsules and tested against white-rot fungi (Trametes versicolor ). The finding obtained from the study indicates that the soybean oil, polystyrene and nanosilver played important roles in the synergistic effect of increasing the decay resistance of Scots pine.

Impregnation of Metallic Nanoparticles for Durability Enhancement

Metallic nanoparticles have been used to protect wood against biodegradation agents and weathering since decades ago [197, 218, 219]. Nanoparticles offer better characteristics than their bulk form, mainly because of their reduced size that leads to high specific surface area-to-volume ratio, uniform size distribution and good stability. Due to its very small size, nanoparticles can penetrate deeply and uniformly into the wood pores leading to a protection of wood [197, 220]. In addition, dispersion stability is improved because of the size and also by addition of surfactant [221]. Dispersion stability coupled with small particle size may greatly improve the following aspects:(1) preservative penetration, (2) treatability of wood, (3) stability of finishes and coating products, (4) low viscosity, and (5) non-leachability [222, 223]. Besides, it can enhance compatibility with binders thereby being able to increase the affinity with wood polymers [197, 224].

Metallic nanoparticle can be prepared by altering the particulate size of metal via chemical reactions, mechanical treatment, heating or refluxing. To date, a lot of nanoparticles have been used for wood protection. Metallic nanoparticles mainly copper, silver, boron and zinc exhibited a good performance against white-rot and brown-rot but less efficient against mould [219]. 이산화티타늄(TiO₂ ) is another nanoparticle that has a good potential to be used as wood preservative. The potential of TiO₂ is related to its antibacterial and antifungal [14, 225] and UV-resistant properties [225, 226]. However, the study on wood treated with TiO₂ is still under preliminary investigation [14, 225]. Table 4 lists the works on the utilisation of TiO₂ for wood preservation. The distribution of TiO₂ in wood was studied by Mohammadnia-Afrouzi et al. (Fig. 3).

SEM images taken from the tangential section of nano-TiO₂ wood treated samples:the 0% moisture content (MC)/0.5% concentration (a1), the 25% MC/0.5% concentration (A2), the 0% MC/1% concentration (A3), the 25% MC/1% concentration (A4), the 0% MC/1.5% concentration (A5) and (25% MC/1.5% concentration. Source :Afrouzi et al. [227]

Copper is an essential biocide for wood protection. However, copper alone fails to protect wood against copper-tolerant wood destroying fungi. Copper nanoparticles is a new generation of wood preservative-based copper. The use of copper nanoparticles instead of conventional copper shows improved durability of wood against decay fungi [230]. This shows that copper nanoparticles can be used to protect wood without the presence of chromium and arsenic [230].

Cristea et al. [15] studied the effects of the addition of ZnO nanoparticles and silver nanoparticles into exterior wood coatings. The purpose of the study is to improve the durability and wood protection through UV shielding. Besides providing an efficient protection against UV, the mechanical properties of wood such as hardness, adhesion strength, the abrasion resistance and the barrier effect for water vapor diffusion were slightly improved.

The mixture of ZnO nanoparticles with silver nanoparticles was able to protect wood from weathering problem such as UV rays [231]. They impregnated sapwood of cottonwood using three different concentrations of mixture by full-cell process. The samples were then exposed to natural weathering. The colour changes of treated wood samples were measured using spectrophotometer. The wood treated with ZnO nanoparticles alone was used as a control. The finding indicates that wood treated with the mixture of ZnO nanoparticle and silver nanoparticles has the lowest colour changes compared to the samples treated with each metallic nanoparticles.

In another study, Mantanis et al. [197] treated black pine wood with ZnO, zinc borate and copper oxide nanoparticles under vacuum. They used acrylic emulsion to force the metallic nanoparticles into the wood structure to avoid leaching. The durability of treated wood against mould decay fungi and subterranean termites was evaluated. Results showed that wood treated with zinc borate slightly inhibited the mold, while the other metallic nanoparticles did not exhibit mould. A similar finding was reported by Terzi et al. [232] that ZnO nanoparticles did not exhibit the mould growth on wood specimens. However, all metallic nanoparticles significantly inhibited white-rot fungi and termite.

Another study shows that metallic nanoparticles are able to improve fire retardant properties of wood. Francĕs et al. [16] studied the effect of SiO₂ , TiO₂ and ZnO₂ infiltrated into pine veneers. They reported that veneer treated using 3 wt% of SiO₂ was most effective to improve the fire retardant behaviour.

Despite the remarkable advantages of nanotechnology in the wood preservation sector as discussed above, the fundamental understanding on synthesis, processing and characterisation of nanocapsules and metallic nanoparticles for wood protection still needs to be improved. The most interesting characteristics need to be considered during the design and development of nanocapsules or metallic nanoparticles for wood protection are as listed in Table 5.

Potential of Nanocellulose-Based Material in Energy Sector

Energy is an important resource that has a strong correlation between economic growth and development [233]. Today, the main energy sources are from fossil fuels and hydroelectric sources, which are very harmful to environment because they can cause climate change and global warning as well as ozone layer depletion, pollution, greenhouse gases emission and ecological destruction [234,235,236]. About 80% of carbon dioxide (CO₂ ) emissions in the world are from energy sector and technology advancement is required to develop sustainable renewable energy resources to reduce the CO₂ emissions as well as to overcome the global warming impact on life and health in line with the needs of accelerating technology development [234, 237].

In order to minimise the environmental effects, a sustainable and low-cost energy efficient carbon-based material has been explored as a potential to replace some conventional materials in the fabrication of energy devices. One of the natural carbon-based materials is cellulose which is the most promising natural polymer with many usages, including energy [237].

Nanotechnology is one of the advanced technologies that have the potential and prospect to fulfil the demand to create clean and green energy. Developing this new material in nanoscale enables new application and its interaction with current energy technology that would revolutionise the energy field from usage to supply, conversion to storage and transmission to distribution [238]. By adapting this nanotechnology, it will have high impact on the development of clean and green energy and benefits the environment and natural resources [239].

According to Serrano et al. [234] and Hut et al. [235], most promising application of nanotechnology for energy conversion is mainly focused on solar energy, conductive materials, solar hydrogen, fuel cells, batteries, power generation and energy devices. Understanding the structural and morphological properties of nanomaterial is essential to obtain the proficiency and sustainability for many applications. The greatest application of nanotechnology in energy generation is solar energy using photovoltaic (PV) cells which focuses on harnessing efficiency [233]. Consumption of energy generated from this solar cell using natural resources will reduce the usage of fossil fuel and decrease the pollution towards creating environmentally friendly and green energy [239]. In addition, the development of nano devices using solar cell could improve the existing materials efficiency as well as reducing manufacturing cost that might increase the economic growth [236].

Nanotechnology has been used in various applications to improve the environment, to solve humanity problems and to produce more efficient and cost-effective energy, such as generating less pollution during products manufacturing, producing solar and fuel cells at a competitive cost, hydrogen production, cleaning up organic chemicals polluting groundwater and overcoming the problem of energy sufficiency, climate change and diseases as well as to reduce the dependency on non-renewable energy sources [233, 235, 240].

Nanocellulose-Based Material for Solar Energy

Solar energy is available in various parts of the world and can be captured from the sun with 15,000 times more energy yearly. This energy source can be used in different ways:photovoltaic (PV) technology, solar thermal systems, artificial photosynthesis, passive solar technologies and biomass technology, which are used to produce electricity, steam or biofuels [234, 235].

In future, nanotechnology might contribute to develop an effective and low-cost system for production, storage, and transporting of energy [235]. According to Serrano et al. [234] and Hut et al. [235], current photovoltaic (PV) market is based on silicon wafer-based solar cells (first generation) and thin film layers of semiconductor materials (second generation). Current drawback of using solar cells is the cost of manufacturing mainly on the high cost of conventional PV cells with poor energy absorption efficiency (less than 40%) [233].

Nanocellulose shows a good potential to be used in the solar energy system due to its renewability, biodegradability, biocompatibility, broad modification capacity, adaptability and versatile morphology [241, 242]. Low cost, flexible and porous substrate of cellulose could be used to produce solar cells. Nanofibrillated cellulose (NFC) with size as low as 4 nm could become the excellent candidate for production of ultrathin paper for use in solar cell component to store the energy [235]. Klochko et al. [243] used nanocellulose from biomass for the development of biodegradable eco-friendly flexible thin film as a thermoelectric material. The thin film was used to convert low-grade waste heat from sun radiation into electricity at near-room temperature.

Nanocellulose-Based Conductive Materials

Conductive materials allow the flow of electrical current which is needed in the fabrication of energy devices. There are many types of conductive materials such as conductive polymers and conductive carbon materials (e.g. carbon nanotubes, graphene, and carbon black) and metallic particle with different levels of conductivity. These conductive materials can be combined with nanocellulose to form novel composites. The process of production conductive nanocellulose is shown in Fig. 2. There are two major strategies involved in nanocellulose based conductive hybrids fabrication process; one is coating of conductive materials layer on the surface of nanocellulose substrates, and another one is mixing the conductive materials inside the nanocellulose substrate to make composite [237]. Conductive polymers are an alternative to metallic materials because of their good electrochemical performance, light in weight and low cost. One of the most promising conductive polymers is polyaniline (PANI) because of its simple route of synthesis, controllable conductivity and high specific capacitance [244].

Nanocellulose-based conductive materials are developed for supercapacitors and energy storage device applications using various types of method such as in situ polymerisation, doping, coating, inkjet printing and in situ depositing [68, 244,245,246]. Modification of the existing supercapacitor by adding nanocomponents has increased its ability to store large amount of energy with longer time of supply [238].

Besides that, nanocellulose-based composite membrane electrodes can be developed via in situ polymerisation of nanocellulose using conductive components via a simple filtration unit (Fig. 3). A well-mixed conductive materials/nanocellulose composite membrane is left on the filter after the liquid has passed through the filter and air-dried composite membrane can be peeled off from the filter membrane for further use as supercapacitors [244].

Nanocellulose for Energy Storage

The potential application of nanocellulose for energy storage application has gained much attention recently. This is due to its nanoscale dimension, high surface area-to-volume ratio, and rich with hydroxyl group, which make their surface chemistry easily modifiable for composite processing. The most important aspect in energy storage is to develop nanocellulose with conductivity and flexibility properties. It can be achieved by adding conducting polymers such as polyaniline and polypyrrole. For example, the nanocellulose/polyaniline composite film is widely used as paper based sensors, flexible electrode, and conducting adhesive [68, 247,248,249]. Razaq et al. [250] manufactured electrodes from the composite of nanocellulose/polypyrrole and carbon filament for paper based energy storage devices. Wang et al. [248] reported that their devices which developed using composite of nanocellulose/polypyrrole provide high charge and discharge rate capabilities, high cell capacitances, and cycling performance.

Nanocellulose-Based Materials for Lithium and Vanadium Battery

High demand on flexible portable electronic devices recently such as smart phone, electrical vehicles, laptops, and even the grid energy storage causes increasing demand on rechargeable lithium-ion battery (LIBs) [251] and supercapacitors [252]. LIBs are one of the most ideal energy storage candidates for electronic devices, due to their high energy density, moderate power density and cycle stability. In LIBs, electrolyte is important for lithium-ion (Li ⁺ ) transfer between anode and cathode. Organic liquid electrolyte is used for LIBs system, but it can pose tremendous safety concern due to the high toxicity and flammability. Solid-state electrolyte has become of interest because it demonstrates obvious advantages of low flammability and low toxicity [253, 254]. According to Janek et al. [255], solid-state electrolyte is classified as solid polymer electrolyte (SPE) and inorganic solid electrolyte (ISE). However, SPE offers the advantages of easy processing and flexibility [255].

Qin et al. [256] developed SPE by incorporating polyethylene oxide (PEO) with nanofibrillated aerogel and bis(trifluoromethanesulphonyl)imide lithium salt (LiTFSI). The results showed that the ionic conductivity properties of SPE were significantly enhanced due to the negatively charged nanofibrillated cellulose. The results also proved that the fabricated SPE is electrochemically stable, mechanically robust and thermally stable as well as flexible, expected for use in flexible electronic devices.

Nair el at. [257] fabricated nanocellulose-laden composite polymer electrolyte for high performing lithium-sulphur batteries using a thermally induced polymerisation method. The composite polymer electrolyte demonstrates excellent ionic conductivity, thermal stability up to more than 200 °C and stable interface towards lithium. The electrolyte also has stable cycling profiles which are attributed to significant reduction of the migration of polysulphide towards anode by entrapment of nanocellulose in the polymer matrix.

Another study was carried out by Zhang et al. [258] on robust proton exchange membrane developed using sulphonated poly(ether sulphone) reinforced by core–shell nanocellulose for vanadium redox flow batteries (VRFBs). It was found that with the incorporation of silica–encapsulated nanocellulose, the proton exchange membrane exhibits outstanding mechanical strength of 54.5 MPa and high energy efficiency above 82% at 100 mA cm ⁻² , which is stable during 200 charge–discharge cycles. Proton exchange membrane is one of the key components in VRFBs. It functions as the separator to avoid vanadium ions crossover. It also acts as proton conductors that contribute to high voltage efficiency in VRFBs [259].

Nanocellulose-Based Materials for Flexible Supercapacitor

Supercapacitor or known as electrochemical double-layer capacitor or ultracapacitor is another versatile energy storage system that has gained the attention of researchers worldwide [260]. With the high power density, long life cycle, simple principles, low maintenance, portability, stable performance and fast charge/discharge rate make supercapacitors capable of filling the gap between batteries and conventional capacitors [260]. These properties offer a promising approach to meet the growing power demands.

Electrode is a very important component in supercapacitor. It requires good electrochemical performance and flexibility especially for preparing a high-performance flexible supercapacitor. Him et al. [261] and Zhe et al. [262] found that graphene and nanocellulose are excellent flexible electrode materials for supercapacitors. Nanocellulose has been used as a substrate material because of its good biodegradability, mechanical strength, flexibility, and chemical reactivity. The porous structure and hydrophilicity of nanocellulose can facilitate the attachment of other materials for example graphene in their fibrous network structure [263]. At the same time, the abundance of hydroxyl groups on the nanocellulose surface enables the interaction of nanocellulose with other polymer to form strong composites [264].

Khosrozadeh et al. [265] developed an electrode for supercapacitor using nanocellulose-based polyaniline/graphene/silver nanowire composite and after applying it for 2400 cycles, at a current density of 1.6 A/g, the supercapacitor showed a power density, energy density and capacitance of 108%, 98% and 84%, respectively. This shows that the electrode has an excellent cyclic stability and good mechanical flexibility. On the other hand, Ma et al. [266] developed an electrode using bacteria cellulose/polypyrrole coated with graphene. The prepared electrode has a good mechanical flexibility in which it can bend at any angle. The area capacitance and energy density can reach 790 mF cm ⁻² and 0.11 mWh cm ⁻² , respectively, when assembled into symmetric supercapacitors. The nanocellulose-graphene electrode can be fabricated using chemical cross-linking or physical cross-linking method [267, 268]. Nanocellulose as an electrode component of supercapacitor also plays the role of internal electrolyte reservoir. This is because nanocellulose can provide an effective way for ion transport. The high pore structure and hydrophilic properties of nanocellulose make it easier to transport electrolyte ion [261].

Nanocellulose-Based Paper for Electronic Devices

Nanocellulose-based paper is a green substrate that can be used for electronic and optoelectronic devices. Currently, commercial paper has relatively rough surface and weak mechanical properties which can be quite problematic for electronic device fabrication [269]. Most of the electronic device’s fabrication use non-biodegradable and non-recyclable component such as plastics, glasses and silicones as substrates [270]. Formation of nanocellulose-based paper from NFC via simple filtration method can produce mechanically strong and low coefficient thermal expansion paper as an alternative to commercial paper [6, 269]. According to Li and Lee [270], transparent nanocellulose-based paper for electronic devices has been designed and being applied for electrochromic, touch sensor, solar cells, transistors, organic light-emitting diodes (OLEDs), gravure printing proofer and radio-frequency identification (RFID).

One of the applications of transparent nanocellulose paper is for the production of flexible electronics materials through printing circuit directly on the surface of substrates via coating or thermal deposition techniques. Firstly, the fabrication of flexible electronics was built on special silicon wafer that allows the silicon nanomembranes (Si NMs) to be released and then transferred to NFC substrate with an adhesive layer, and the device was completed by photoresist patterning and dry etching steps as illustrated in Fig. 4 [271].

Schematic illustration of the generalised fabrication routes to nanocellulose-based conductive hybrid. Source :Du et al. [237]. Reproduced with permission of Elsevier

Moreover, NFC also is the main substrates for organic light-emitting diodes (OLEDs) (Fig. 5). Okahisa et al. [272] reported that the OLEDs device was fabricated on wood-based nano fibrillated cellulose (NFC) composite.

Filtration procedure to fabricate nanocellulose-based composite membrane [supercapacitor (SC)]. Source :Hsu and Zhong [244]

Potential of Nanocellulose-Based Composite for Development of Sensor

Nanocellulose has been widely used to develop novel sensors and improve the sensitivity of sensors. In food industry for example, sensors have become an important tool to protect humans from health hazards and risks caused by the food contaminants. Sensors can help to quickly identify mycotoxins, pathogens, heavy metals, pesticides, metal ions, and so on in food. In addition, sensor technology can overcome the complicated, laborious, and time consuming process using expensive instrument that usually requires well-trained personnel [273, 274]. Sensor also provides rapid and sensitive food safety detection.

In the last decade, plenty of sensors (electrochemical sensors, biosensors and chemical sensors) have been successfully developed as alternative or as complementary detection tools for the rapid and sensitive detection [275,276,277]. However, conventional sensors were developed using plastics, petrochemical-based products and inorganic material which lead to environmental problems. Moreover, issues such as green gas emission, toxicity and sustainability of the materials are becoming increasing. Therefore, the demand for sustainable sensor devices has increased rapidly in recent years.

Nanocelluloses from plants and bacteria have shown promising potentials due to their excellent physical, thermal, mechanical, optical and physical properties, which are important for fabricating high-performance sensor devices. These properties make nanocellulose more preferred as biomaterials since it can enhance the selectivity and sensitivity of sensors for the detection of analytes. On the other hand, the adhesion properties of nanocellulose prevents the leaching problems of immobilised reagents. Thus a remarkable improvement can be obtained for the long-time stability of the sensor. Besides, the hydroxyl –OH groups on nanocellulose can be modified for the incorporation of binding sites for the selective adsorption of different analyte species [278]. The combination of those characteristics makes the nanostructured nanocellulose fibres an ideal building block for conjugation with other functional materials [248, 279, 280] (Table 6).

Development of Biosensors

Kim et al. [286] immobilised enzymes candida rigosa lipase into the different cellulose nanocrystals. The findings indicate candida rigisa lipase absorbed on nanocellulose was relatively higher compared to that of microcellulose. This can be related to the high surface area on nanocellulose and thus increase the ionic interaction between nanocellulose anionic group and candida rigisa lipase. In this study, they found that the half-life of the candida rigisa lipase immobilised in nanocellulose increased 27 times higher compared to in free form. Besides, the stability of enzyme also increased (Fig. 6).

Flexible electronics production process on NFC substrate. Source :Sabo et al. [271]

Another group of researchers, Edwards et al. [287] studied the kinetic profiles of tri- and tetrapeptides substrate of elastase for the fabrication of elastase biosensor whereby known as human neutrophil elastase (HNE) and porcine pancreatic elastase (PPE). To develop HNE and PPE, immobilised tri- and tetrapeptides were used into cotton cellulose nanocrystals. They found that 2 mg of tripeptide conjugated cotton cellulose nanocrystals in 1 h was able to detect 0.03 U mL ⁻¹ PPE, while 0.2 mL tetrapeptide conjugated cotton cellulose nanocrystals over 15 min could detect 0.05 U mL ⁻¹ HNE activity. Incani et al. [288] fabricated a biosensor by immobilising glucose oxidase (GOx) enzyme in a nanocellulose/polyethyleneimine (PEI)/gold nanparticles (AuNPs) nanocomposites. The AuNPs were adsorbed on the cationic PEI and nanocellulose. The Fourier Transform Infrared (FTIR) spectra confirmed that GOx was successfully immobilised on the polymer composites (Fig. 7).

Flexible display (OLEDs) on NFC substrate, a Source :Okahisa et al. [272]. Reproduced with permission of Elsevier. Flexible electronics on NFC substrate, b Source :Sabo et al. [271]

Abd Manan et al. [289] successfully developed biosensor based on nanocrystalline cellulose (NCC)/cadmium sulphide (CdS) quantum dots (QDs) nanocomposites for phenol determination. They modified the NCC with cationic surfactant of cetyltrimethylammonium bromide (CTAB) and further decorated with 3-mercaptopropionic acid (MPA) capped CdS QDs as a scaffold for immobilisation of tyrosinase enzyme (Tyr). The TEM images of NCC and CTAB-NCC (Fig. 8a, b) indicates that agglomerated whiskers-like structure of NCC was not affected by modification, while the MPA QDs are of spherical shape (Fig. 8c). The FESEM images of CTAB-NCC nanostructured film (Fig. 8d) exhibited a homogenous, uniform and dense fibrous structures aggregated, while for CTAB-NCC/QDs nanocomposites film (Fig. 8e), the CdS QDs were appeared as like tiny white dots. EDX analysis shows the presence of respective elemental of carbon (C), oxygen (O), sulphur (S) and cadmium (Cd), indicating the CdS QDs was successfully attached into the CTAB-NCC film. The test finding against phenol shows the biosensor exhibits good linearity towards phenol in the concentration range of 5–40 µM (R2 = 0.9904) with sensitivity and limit of detection of 0.078 µA/µM and 0.082 µM, respectively.

The TEM images for a NCC, b CTAB-NCC, c MPA-QDs, the FESEM micrograph of d CTAB-NCC nanostructures film, e CTAB-NCC/QDs nanocomposites film, f EDX analysis of CTAB-NCC nanostructured film, g CTAB-NCC/QDs nanocomposite film (Source :Abd Manan et al. [289]). Reproduced with permission of Elsevier

In the food industry, sensors are used to monitor the quality or freshness of food. Moradi et al. [290] developed a pH sensing indicator based on bacterial cellulose nanofibres for monitoring freshness of fish. They used carrot anthocyanins as an indicator. The fabricated sensor displayed wide colour differences from red to gray over the 2–11 pH range. The colour changes were distinguishable whereby deep carmine colour indicates fresh fish, charm pink colour indicates fish is best to eat immediately, and spoiled fish is indicated by jelly bean blue and khaki colours.

Development of Chemical Sensors

Nanocellulose hydrogels structure was previously used to immobilise sulphur and nitrogen co-doped graphene quantum dots as a low-cost sensor for detecting the laccase enzyme [291]. Laccase is the multicopper oxidase family of enzyme. It is involved in the monoelectronic oxidation of several aromatic compounds and aliphatic amines. Therefore, it is usually used in decolouration and coloration products [292, 293]. Therefore, monitoring of laccase activity in commercial products is of great interest. From this study, the authors found the sulphur and nitrogen co-doped graphene quantum dots were self-organised via electrostatic interactions.

Faham et al. [294] described the development of a nanocellulose-based colorimetric assay kit for smartphone sensing of iron, Fe(II) and iron-chelating deferoxamine drug (DFO) in biofluids. They embed curcumin in a transparent bacterial cellulose nanopaper, as a colorimetric assay kit. The assay kit was then used for monitoring of iron and deferoxamine as iron-chelating drug in biological fluid such as urine, blood, saliva and serum. The assay kit can be easily coupled with smartphone technology for colorimetric monitoring of Fe(II) and DFO.

Development of Electrochemical Sensors

Burrs et al. [295] demonstrated the electrochemical biosensor-based glucose for the detection of pathogenic bacteria. The conductive paper used for the development of electrochemical sensor was prepared using graphene-nanocellulose composites. Platinum was electrodeposited on graphene-cellulose composites using pulsed sonoelectrodeposition [296]. Then the sensor was fabricated by functionalising the nanoplatinum with glucose oxidase (GO_x ) entrapped in a chitosan hydrogel on the conductive paper. They found that the sensor is extremely efficient for use in electrochemical biosensing with a low detection limit for glucose or pathogenic bacteria.

Ortolani et al. [297] developed an electrochemical sensing device using nanocellulose and single-walled carbon nanohorns (SWCNH) for guanine and adenine determination. Both nanocellulose and SWCNH have large surface area, good conductivity, high porosity and chemical stability. The prepared sensor showed highly sensitive and high electrocatalytic activity towards simultaneous determination of guanine and adenine. The sensor also presented a lower limit detection. Another study conducted by Shalauddin et al. [298] used hybrid nanocellulose/functionalised multi-walled carbon nanotubes (f -MWCNTs for development of electrochemical sensing. The sensor for determination of diclofenac sodium in pharmaceutical drugs and biological fluids samples were used. The presence of –OH groups in the nanocellulose was reported to provide more binding sites for different analytes which ensures an axial modulus rearrangement and incorporation of f -MWCNTs.