Nanocellulose as a promising substrate for advanced sensors and their applications

Abstract

Nanocellulose has broad and promising applications owing to its low density, large specific surface area, high mechanical strength, modifiability, renewability. Recently, nanocellulose has been widely used to fabricate flexible, durable and environmental-friendly sensor substrates. In this contribution, the construction and characteristics of nanocellulose-based sensors are comprehensively reviewed. Various nanocellulose-based sensors are summarized and divided into colorimetric, fluorescent, electronic, electrochemical and SERS types according to the sensing mechanism. This review also introduces the applications of nanocellulose-based sensors in the fields of biomedicine, environmental monitoring, food safety, and wearable devices.

Introduction

Cellulose, which is biosynthesized in plants and forms partially crystalline fibers, is one of the most important natural polymers. Cellulosic materials with at least one dimension in the nanometer range are referred to as nanocellulose. In general, depending on the morphological characteristics, functions, and preparation methods (mainly depending on the source and processing conditions of cellulose materials), nanocellulose is divided into three categories: cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial nanocellulose (BC) (Fig. 1). CNCs and CNFs are obtained via a top-down approach consisting of the disintegration of plant matter via chemical or mechanical treatment. Mechanical shearing or acid hydrolysis will first weaken and destroy the regions with the least crystallization to yield the expected nanocellulose [1]. CNCs are high-purity whiskers with rod-like shape, and have high rigidity because the amorphous regions are removed to a great extent via acid hydrolysis. CNFs are usually produced/extracted by mechanical disintegration processes, such as delamination of lignocellulosic fibers before and/or after enzymatic or chemical treatment [2], [3]. The CNFs, which have widths in the range of nanometers and lengths of up to several micrometers, are formed during cellulose biosynthesis [2], [3], [4], [5], [6], [7], [8], [9], [10]. In the case of CNF the main large-scale applications include papermaking strength additives and composite applications but also in the fields of rheological modifiers (e.g., food and paint applications) and the emerging fields of electronic and cosmetic applications and nanofilter devices. CNCs are most destined as interface stabilizers, rheological modifiers, as films/coatings and reinforcing agents in polymer composites but also are non-toxic and show great potential in biomedical devices as well [11]. Compared to CNFs, BC demonstrates better purity, crystallinity and mechanical stability Different from CNCs and CNFs, BC is produced via a bottom-up approach using cultures of bacteria to synthesize the material. The biological formation of BC provides an opportunity to develop biotechnological production pathways to significantly affect and control the final BC material features. Compared to CNFs, BC demonstrates better purity, crystallinity and mechanical stability [12]. Nanocellulose possess unique structure, morphology and properties, allowing it to be used in various fields, such as reinforcement, packaging, environmental remediation, sensing detection, biomedicine, porous magnetic aerogels, display substrates, photocatalysis, water treatment, and etc. [11], [13], [14], [15], [16], [17], [18], [19], [20]

Sensor is a detection device that can respond to physical, chemical and biological changes in the environment and convert them into electrical signals or other required signal output according to certain rules to meet the requirements of information transmission, processing and storage. The most advanced materials used in conventional sensors are semiconductors, metals, and metal oxides, but their poor stretchability and brittle mechanical properties are essentially not suitable for flexible sensors [24]. Extensive research work has focused on establishing fungible materials and the fabrication strategies to avoid these disadvantages in mechanical properties without compromising functionality or performance advantages. Common materials for fabricating flexible sensors are environmentally unfriendly synthetic polymers, such as polydimethylsiloxane (PDMS) [25], polyurethane (PU) [26], polyimide [27], [28] polyvinyl alcohol [29], polyglycerol sebacate [30], polyethylene glycol [31], and etc.

Owing to the renewability, biodegradability, biocompatibility, high mechanical strength, large specific surface area, low visual light scattering, thermal stability, nanocellulose is an excellent host matrix and biological template/ scaffold to fabricate organic/ inorganic composite sensors with high strength, durability and flexibility. Various types of guest materials with excellent electrical, optical, and mechanical properties, including molecules, polymers, and nanoparticles, have been successfully immobilized on nanocellulose [12], [18], [32], [33], [34], [35], [36], [37], [38], [39], [40]. These nanocellulose-based sensors can detect target analytes via colorimetric assay, fluorescence, electrochemical signal, surface enhanced Raman scattering spectroscopy (SERS), and etc. [27]. In addition, nanocellulose has abundant active groups, which can be modified to improve the sensitivity and accommodate binding sites to selectively adsorb analyte species [41], [42]. In short, nanocellulose is an excellent sensor substrate for a variety of sensing applications [12], [43], [44], [45].

In the past decade, the development of nanocellulose-based sensors has made great progress. Edwards et al. first classified the application of CNCs in biosensors and focused on the combination of CNCs with biologically active molecules (i.e., peptides and proteins) for biosensor design [43], [46]. Years later, Golmohammadi et al. reviewed the application of nanocellulose in optical, bioimaging, and electrical sensing [12]. SERS substrates based on nanocellulose as the useful platforms for sensing applications were also reviewed [47], [48]. Nowadays, skin biocompatible products are fast-growing markets for nanocelluloses with increasing number of patents published in last decade. Over the past decades, nanocelluloses have dramatically evolved as highly functional and biocompatible materials for applications onto the skin e.g., skincare, cosmetics, healthcare and health monitoring. However, most of these reviews were published four years ago. In addition, the introductions of nanocellulose-based sensors in above reviews are not comprehensive enough, especially the classification and detection mechanism. In this contribution, the construction strategy, structure characteristics, target analyte, sensing mechanism and performance of six main nanocellulose-based sensors are summarized, especially the advantages of nanocellulose in the substrate fabrication. In addition, the applications of nanocellulose-based sensors in various fields are comprehensively reviewed.