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Bio-inspired materials derived from cellulose nanocrystals for anti-counterfeiting systems
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O comércio de contrafações num número crescente de indústrias tornou-se um grave problema na sociedade atual, pondo em risco a saúde e segurança pública. Recentemente, o estudo de materiais baseados em nanocristais de celulose tem gerado muito interesse por parte da comunidade científica, dado que estes materiais ecológicos apresentam propriedades únicas adequadas para aplicações fotónicas e de anti-contrafação.
Filmes sólidos produzidos a partir de suspensões aquosas liotrópicas de nanocristais de celulose (CNCs) podem reter o arranjo colestérico, exibindo cores estruturais e outras propriedades óticas notáveis como iridescência, reflexão seletiva de luz circularmente polarizada à esquerda e transmissão seletiva de luz circularmente polarizada à direita, assemelhando-se ao que é encontrado na cutícula de certos insetos e frutos. Estas propriedades biomiméticas dos CNCs têm grande potencial para serem usadas para encriptação ótica como medida de anti-contrafação, com a iridescência garantindo uma forma de autenticação visível, e a reflexão de luz circularmente polarizada à esquerda oferecendo um nível adicional de encriptação invisível.
O principal objetivo deste trabalho é a produção de filmes nanocompósitos compostos por CNCs e hidroxipropilmetilcelulose (HPMC) que apresentem propriedades óticas adequadas para serem utilizados como elementos de segurança em sistemas de anti-contrafação. Os efeitos da concentração de CNCs e do tempo de hidrólise ácida na formação da fase colestérica e nas propriedades óticas de filmes gota sólidos foram primeiramente investigados em reações de “scale-up”, revelando que um aumento de concentração de CNCs resulta numa alteração das cores refletidas pelos filmes para a zona do vermelho do espectro visível. Foram produzidos filmes nanocompósitos com diferentes rácios CNCs/HPMC, e foi realizado um estudo sobre a influência do contra-ião dos CNCs nestes filmes. Verificou-se que a adição de pequenas quantidades de HPMC às suspensões líquidas cristalinas induzia uma alteração das cores refletidas pelos filmes para a zona do vermelho, enquanto que incrementos acima de 5 wt. % de HPMC geravam uma alteração para a zona do azul, mantendo a organização estrutural e melhorando a flexibilidade dos filmes. Estes resultados sugerem uma abordagem inovadora para usar materiais naturais em dispositivos de segurança, com a possibilidade de manipulação das propriedades óticas usando quantidades mínimas de material.
The trade of counterfeits in a growing number of industries has become a severe problem in today’s society, jeopardizing public health and safety. Recently, the scientific community has been extremely interested on the study of cellulose nanocrystals-based materials, as these eco-friendly materials present unique properties suitable for photonic and anti-counterfeiting applications. Solid films produced from lyotropic aqueous suspensions of cellulose nanocrystals (CNCs) can retain the cholesteric arrangement, displaying outstanding optical properties such as iridescent structural colours, selective reflection of left-handed circularly polarised (LCP) light and transmission of right-handed circularly polarised (RCP) light, resembling what is found in certain insect cuticles and plant fruits. These bio-mimicking optical properties of CNCs films offer great potential to be used for encryption as an anti-counterfeiting measure, with iridescence granting an overt form of authentication and LCP light reflection offering an additional level of covert encryption. The main goal of this work is the production of nanocomposite films with CNCs and hydroxypropylmethylcellulose (HPMC) that present suitable optical properties to be employed as security features in anti-counterfeiting systems. The effects of CNCs concentration and acid hydrolysis time on the formation of the cholesteric phase and on the optical properties of drop-casted films were firstly investigated in scale-up reactions, revealing that increased CNCs concentrations give rise to a red-shift on the films’ reflected colours. Nanocomposite films with different CNCs/HPMC ratios were produced, and the influence of the CNCs counterion in these films’ properties was studied. The addition of small amounts of HPMC to the liquid crystalline suspensions was found to induce a red-shift on the films’ reflected colours, while increments above 5 wt. % of HPMC generated a blue-shift, maintaining their structural organization and enhancing their flexibility. These findings propose an innovative approach for using naturally-occurring materials in security devices, enabling manipulation of optical properties using minimal quantities of material.
The trade of counterfeits in a growing number of industries has become a severe problem in today’s society, jeopardizing public health and safety. Recently, the scientific community has been extremely interested on the study of cellulose nanocrystals-based materials, as these eco-friendly materials present unique properties suitable for photonic and anti-counterfeiting applications. Solid films produced from lyotropic aqueous suspensions of cellulose nanocrystals (CNCs) can retain the cholesteric arrangement, displaying outstanding optical properties such as iridescent structural colours, selective reflection of left-handed circularly polarised (LCP) light and transmission of right-handed circularly polarised (RCP) light, resembling what is found in certain insect cuticles and plant fruits. These bio-mimicking optical properties of CNCs films offer great potential to be used for encryption as an anti-counterfeiting measure, with iridescence granting an overt form of authentication and LCP light reflection offering an additional level of covert encryption. The main goal of this work is the production of nanocomposite films with CNCs and hydroxypropylmethylcellulose (HPMC) that present suitable optical properties to be employed as security features in anti-counterfeiting systems. The effects of CNCs concentration and acid hydrolysis time on the formation of the cholesteric phase and on the optical properties of drop-casted films were firstly investigated in scale-up reactions, revealing that increased CNCs concentrations give rise to a red-shift on the films’ reflected colours. Nanocomposite films with different CNCs/HPMC ratios were produced, and the influence of the CNCs counterion in these films’ properties was studied. The addition of small amounts of HPMC to the liquid crystalline suspensions was found to induce a red-shift on the films’ reflected colours, while increments above 5 wt. % of HPMC generated a blue-shift, maintaining their structural organization and enhancing their flexibility. These findings propose an innovative approach for using naturally-occurring materials in security devices, enabling manipulation of optical properties using minimal quantities of material.
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Palavras-chave
cellulose nanocrystals cholesteric phase hydroxypropylmethylcellulose nanocomposites nanocomposites circularly polarised light