Nanocellulose-based Surface-enhanced Raman spectroscopy sensor for highly sensitive detection of TNT

Highlights

• SERS substrates with 3D hot spots were developed based on nanocellulose aerogels.

• Self-assembly of core-shell Au@Ag nanocubes and Meisenheimer complex strategy were adopted.

• Highly-sensitive and reproducible SERS detection of TNT was achieved.

Abstract

Surface-enhanced Raman scattering (SERS) has undergone an important development in the detection and identification of extremely low traces of explosives over the past few years. Here we fabricated a new type of three-dimensional (3D) SERS sensors by loading AuNR@AgNCs on bacterial cellulose aerogels for the detection of 2, 4, 6-trinitrotoluene (TNT). The aerogel substrates with hierarchical pore structure are favored by the construction of dense 3D hot-spot distribution and the enrichment of TNT molecules. The retention aid of polyethyleneimine was used to improve the loading capacity as well as prevent the aggregation of nanoparticles. Moreover, AuNR@AgNCs were modified with p-aminobenzenethiol to improve the affinity of SERS substrates to TNT via the formation of Meisenheimer complex. Owing to the unique structure and morphology of AuNR@AgNCs, the composite SERS substrate can detect trace TNT with a limit of detection of 8 × 10⁻¹² g/L and the SERS enhancement factor is high up to 1.87 × 10⁸.

Introduction

Detection of trace explosives is a key challenge in both the securing of public spaces and vulnerable targets and the environmental monitoring of groundwater and soil. There is a strong need for the analytical techniques that accurately and quickly detect 2, 4, 6-trinitrotoluene (TNT), which is one of the most dangerous artificial explosives. Current analysis methods are relying heavily on mass spectrometry (Milligan, Shand, Graham, & Faulds, 2020), X, γ-ray imaging technology (Forbes & Sisco, 2018), ion mobility spectrometry (Chiluwal et al., 2019), and terahertz spectroscopy (Trofimov & Varentsova, 2019) Meanwhile, other methods including electrochemical measurement (Yu, DeTata, Lewis, & Silvester, 2017), canine detection (Furton & Myers, 2001), and fluorescence (Tu et al., 2008) are also developed recently. Although these methods yield reliable results, they often require expensive instruments, complex operation, and specially trained personnel. Surface-enhanced Raman spectroscopy (SERS) is a technique used to enhance the Raman scattering signal of objective molecules on a specially prepared noble metal (mainly Au and Ag) (O’Flynn et al., 2013) substrate (Chen, Huang, Kong, & Lin, 2019; Nabeela, Thomas, Mohamed, & Pillai, 2020). It is believed that the intensity of inelastically scattered photons from the molecule can be enhanced using the long-range electromagnetic (EM) field and short-range chemical effects (Zhang, Liu et al., 2018). EM enhancement is attributed to the surface plasmon resonance (SPR) in the near-field metallic surface. And the chemical enhancement is because of the charge transfer (Miao, Lin, & Bian, 2020) mechanism between the substrate and the analyte. In particular, very close interparticle junctions between nanoparticles have shown dramatically improved signal amplification for analytes localized in the nanogap area, which was also presumed to be “hot spots” for SERS (Li et al., 2015). Moreover, the SERS performance of the substrate is closely related to the SPR of noble metal nanoparticles, which is largely affected by factors such as particle size, morphology, type, and arrangement (Wei, Du, Zhang, Yang, & Gao, 2018). Important attributes such as high resolution, narrow bands, fingerprint, no interference of water, photobleaching resistance, real-time, and noninvasive have enabled SERS to find important application in the field of trace explosive detection (Neupane, Oh, Park, & Lee, 2016; Rasheed, Li, Nabeel, Huang, & Zhou, 2019).

The key technology for SERS substrate with ultra-sensitive is to increase the specific surface area to adsorb more molecules as well as maximize the distribution density of hot spots to enhance the Raman signal. There are various developed SERS substrates over the past decade, including one-dimensional (1D) nanowire, two-dimensional (2D) plane array, and three-dimensional (3D) porous network (Wu et al., 2020). Owing to the increasing specific surface area (SSA), more hot spots can be achieved from low to high dimensional SERS substrates, resulting in higher detection sensitivity (Zhang, Xiao et al., 2017; Zhou, Liu, Zhang, Li, & Cai, 2019). Besides, compared to 1D and 2D substrates, the 3D substrate shows better utilization efficiency of the light source because it does not require precise laser focusing and can make full use of the scattered light (Wu et al., 2020). The preparation of SERS substrates has advanced from early approaches using electrochemically roughened metal surfaces to more economical and controllable physical or chemical methods, such as filtration, electron beam evaporation, self-assembly, in-situ synthesis, and etc. (Yang, Jiao, Min, Liu, & Dai, 2017; Yorseng, Siengchin, Ashok, & Rajulu, 2020), to load metallic nanoparticles on specific supporters including porous alumina membrane (Chang, Ko, Singamaneni, Gunawidjaja, & Tsukruk, 2009), carbon nanotube array (Sun et al., 2010), cotton swabs (Gong et al., 2014), diatomite (Kong, Chong, Squire, & Wang, 2018), and etc. However, it is still a challenge to develop environmentally-friendly and low-cost SERS substrates with ultra-sensitive and signal space homogeneity through simple but effective techniques.

Naturally-derived nanocellulose, which possesses high SSA, flexibility, high strength, and tunable surface chemistry, biodegradability, has been demonstrated as ideal materials for constructing SERS substrates (Chen, Ying, Song, & Liu, 2019; Gu & Dichiara, 2020; Miao et al., 2020; Xiong, Lin, Lin, & Huang, 2018). A SERS plate based on nanofibrillated cellulose paper was fabricated through wax-printing-based multiwell patterning followed by in-situ growth silver of nanoparticle (Chen, Ying et al., 2019). The ultrasmooth SERS substrate with 2D hot spots achieved the detection of rhodamine at picomolar concentration. Kwon et al. also developed a nanoporous SERS platform to analyze thiram hazardous pesticides by loading Au nanorods (AuNRs) on cellulose nanofiber paper with nanometer roughness, which allowed for the high-density distribution of hot spots (Kwon et al., 2019). The composite SERS platform exhibited the ability to detect thiram at the concentration as low as 1 nM. However, most of the researches on the nanocellulose-based SERS sensors are focused on the construction of 2D substrates. Bacterial cellulose (BC) is an interesting ribbon-shaped ultrafine cellulose nanofibers produced by Acetobacter xylinum, which has the advantages of high aspect ratio, high Young's modulus, template structure, high surface activity, and biocompatibility (Abbasi-Moayed, Golmohammadi, & Hormozi-Nezhad, 2018; Torres, Arroyo, & Troncoso, 2019). Owing to the high aspect ratio and the large number of surface hydroxyl groups, BC can easily be processed to flexible aerogels with high SSA and 3D network by means of entanglement, hydrogen bonding, chemical crosslinking, etc. (Purwidyantri et al., 2020). BC aerogel (BA) is an excellent supporter to prepare functional composites by immobilizing guest materials, such as molecules, polymers, and nanoparticles (Tian et al., 2016). In this study, we show that 3D SERS sensors of TNT can be easily and cost-effectively fabricated in batches using BA as supporters. For the first time, AuNRs coated with the shell of Ag nanocubes (AuNR@AgNCs) were loaded on the BC network via electrostatic interaction, leading to the formation of 3D hot spots. In order to further improve SERS enhancement, the sensing strategy of the Meisenheimer complex, which is able to combine the substrate with the objective molecule through CT, is used to selectively capture and enrich TNT molecules. The obtained composite SERS substrates with both EM and CT effects possess excellent sensitivity for the detection of trace TNT explosive.