Cellulose-based fluorescent materials for chemical sensing applications

Sensing materials, also referred to as stimulus-response materials, are a variety of functionalized materials that can respond reversibly and visually to various external stimuli (e.g., acids/alkalis, vapors, ions, temperature, humidity, etc.) [1,2]. Among them, the stimuli-fluorochromic materials possess photoluminescent properties that can be switched by imposing diverse environmental stimuli, which have drawn significant attention on account of their highly sensitive responsiveness and convenient visualization [3,4]. Therefore, being a high-performance functional material, stimuli-fluorochromic materials are extensively utilized in sensors [5,6], information storage [7,8], bio-imaging [9,10], information encryption [11,12], smart display [13,14], green printing [15,16] and anti-counterfeiting [17,18]. Sensors based on stimuli-fluorochromic material have been extensively investigated for decades due to the high sensitivity and selectivity as well as their potential for instant monitoring of diverse targets, including metal ions, anions, biomolecules and gas molecules [[19], [20], [21]]. With the emerging advances in sensing technology, traditional solution-state sensing has manifested significant limitations in practical applications [22]. In contrast, solid-state sensing materials provide the benefits of being portable, easy to operate and reusable, thus enabling instant detection in a cost-effective manner [23,24]. These material-based probes can eventually be used as portable measuring devices for field applications [25]. Therefore, it is important to search for environmentally friendly material-based sensors with simple and intelligent detection capabilities [[26], [27], [28]].

As an abundant natural biomass organic polymer, cellulose has good biocompatibility, excellent mechanical properties and high structural designability, which endows it with the status of an ideal platform for fluorescent sensing [[29], [30], [31]]. The abundant hydroxyl groups (-OH) existence on cellulose chains provides abundant potential active sites for the introduction of diverse fluorophores and luminescent moieties. Through chemical bonding, self-assembly and coordination, a variety of cellulose-derived fluorescent materials could be fabricated and have been extensively applied in the fluorescent sensing [[32], [33], [34]]. The fabricated cellulose-based fluorescent sensing materials are equipped with the unique photosensitive properties of fluorescent functional substances, as well as the excellent processability and mechanical properties unique to cellulose substrates [[35], [36], [37], [38]]. Therefore, cellulose-based fluorescent materials could be readily fabricated into various forms, including fluorescent fibers, sensing labels/films, fluorescent hydrogels, fluorescent inks, etc., which vastly broadening the applications of fluorescent sensing materials [[39], [40], [41], [42]]. Cellulose derivatives obtained by structural modification and design acquire additional solubility, thermoplasticity and viscosity in addition to the excellent properties of cellulose [43]. Benefiting from the various physicochemical advantages, cellulose and its derivatives perform a variety of important roles in fluorescent sensing materials. They can be used as substrates to enhance the mechanical property in fluorescent sensing material systems, as binders and stabilizers in fluorescent inks, and as raw materials for the preparation of fluorescent materials, e.g. obtain nanocrystals with unique photonic properties and carbon quantum dots with high quantum yield (QY) [44]. Cellulose-based fluorescent materials are usually fabricated via physical blending and chemical combination of cellulose and fluorescent substances, top-down dissociation of photoactive substances from cellulose, and in-situ microbial synthesis of bacterial cellulose with fluorescent properties [45,46]. Therefore, cellulose-based fluorescent sensing materials are available in diverse preparation approaches and manipulable morphologies, which have greatly facilitated their practical application and shown significant potential in visual sensing and other aspects. Over the past two decades, a great deal of research has been published (Fig. 1).

According to our knowledge, there are few reviews on fluorescent sensing materials related to cellulose. Herein, based on the previous work and the research conducted in our lab, the construction, performance analysis and utilization of cellulose-derived fluorescent sensing materials over the past two decades are reviewed. Strategies for fabricating fluorescent sensing materials based on cellulose are summarized, while the performances and applications of these materials are highlighted (Fig. 2). Finally, an outlook on future research, challenges and perspectives available for cellulose-based fluorescent sensing materials is presented. This review is bound to attract growing attention and offer guidance to chemists, biochemists, and chemical engineers engaged in optical sensor development for forthcoming research.