Chitosan reduced in-situ synthesis of gold nanoparticles on paper towards fabricating highly sensitive, stable uniform SERS substrates for sensing applications

Flexible Surface-Enhanced Raman Scattering Tape Based on Ag Nanostructured Substrate for On-Site Analyte Detection

Surface-enhanced Raman scattering (SERS) has emerged as a powerful surface analytical technique that amplifies Raman scattering signals of molecules adsorbed onto metal nanostructured surfaces. The droplet reaction method has recently been employed to fabricate large-scale microring patterns of silver (Ag) nanostructures on rigid substrates, which enables sensitive detection within the ring area. However, these rigid substrates present limitations for direct on-site detection of analyte residues on irregular sample surfaces. There is a need to develop soft and flexible SERS substrates that can intimately conform to arbitrary surfaces. In this study, we presented a SERS substrate using flexible and adhesive tape as the supporting material. This SERS tape was fabricated by repeatedly transferring presynthesized Ag nanostructures from a rigid substrate to the tape. For a model compound adenine, our SERS tape exhibited a good linear response from 5 × 10-4 M to 5 × 10-5 M with a low limit of detection (LOD) of 5 × 10-7 M and displayed a SERS enhancement factor (EF) of 3.2 × 105. The relative standard deviation (RSD) of SERS intensity achieved was as low as 1.93%, indicating its outstanding uniformity. The as-prepared SERS tape was used for in situ detection of pesticide residue on an apple surface and dye residue on human hair. Leveraging the large surface area of Ag nanostructure patterns from the droplet reaction, the developed SERS tape demonstrates excellent performance in terms of sensitivity and uniformity. The successful detection of analyte residues on arbitrary surfaces of apple and human hair highlights the potential of this flexible SERS tape for real-world applications across various industries for enhanced diagnostic accuracy.

Introducing a portable electrochemical biosensor for Mycobacterium avium subsp. paratuberculosis detection using graphene oxide and chitosan

In this contribution, a novel, low-cost, high throughput, and ultra-selective electrochemical DNA nanobiosensor was developed for accurate on-site detection of Mycobacterium avium subspecies paratuberculosis (MAP) in real media for practical diagnosis of Johne's disease (JD). The method was designed based on the immobilization of graphene oxide and chitosan biopolymer on the surface of a glassy carbon electrode, modified by electrochemical immobilization of graphene oxide and chitosan biopolymer, followed by activation of biopolymer via 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride and N-hydroxy succinimide (EDC/NHS) coupling system. Afterward, the commercial probe DNA (ssDNA) was stabilized on the activated electrode surface to prepare an ultra-selective ssDNA-stabilized nanobiosensor for MAP sensing called "ssDNA-stabilized GO-CH-EDC/NHS-modified electrode". Several characterization methods distinguished the bioelectrode. The DNA hybridization between the nanobiosensor and target DNA was confirmed by cyclic voltammetry and differential pulse voltammetry. "At optimal experimental conditions, the nanobiosensor showed a linear range of 1.0 × 10-15-1.0 × 10-12 mol L-1, a detection limit as low as 1.53 × 10-13 mol L-1, and a repeatability with a relative standard deviation (%RSD) of 4.7%. The reproducibility was also appropriate, with a %RSD of about 10%. It was used to diagnose MAP in real samples with highly accurate results. Therefore, the developed nanobiosensor can be used for clinical diagnosis of MAP.

Tuning Hydrophobicity of Paper Substrates for Effective Colorimetric detection of Glucose and Nucleic acids

This study investigated the colorimetric response of standard glucose, serum glucose, and nucleic acid assays on various paper surfaces with different wettability, including hydrophilic, hydrophobic, and nearly superhydrophobic surfaces. Water contact angles (WCA) formed by water droplets on each surface were measured using ImageJ software. The hydrophilic surface showed no contact angle, while the hydrophobic and nearly superhydrophobic surfaces exhibited contact angles of 115.667° and 133.933°, respectively. The colorimetric sensitivity of the standard glucose assay was analyzed on these surfaces, revealing enhanced sensitivity on the nearly superhydrophobic surface due to the high molecular crowding effect owing to its non-wetting behavior and eventually confined reaction product at the sample loading zone. The hydrophobic nature of the surface restricts the spreading and diffusion of the reaction product, leading to a controlled and localized concentration of the assay product leading to moderate colorimetric intensity. On the other hand, the hydrophilic surface showed the least enhancement in colorimetric sensitivity; this is attributed to the high wettability of the hydrophilic surface causing the reaction product to spread extensively, resulting in a larger area of dispersion and consequently a lower colorimetric intensity. The measured limit of detection (LOD) for nucleic acid on nearly superhydrophobic surfaces was found to be 16.15 ng/µL, which was almost four-fold lower than on hydrophilic surfaces (60.08 ng/µL). Additionally, the LODs of standard glucose and clinical serum samples were two-fold lower on nearly superhydrophobic surfaces compared to hydrophilic surfaces. Our findings clearly highlight the promising potential of utilizing superhydrophobic surfaces to significantly enhance colorimetric sensitivity in paper-based diagnostic applications. This innovative approach holds promise for advancing point-of-care diagnostics and improving disease detection in resource-limited settings.

Sensitive determination of thiram in apple samples using a ZIF-67 modified Si/Au@Ag composite as a SERS substrate

Substrate materials with high sensitivity and storage stability are crucial for the practical analytical application of surface-enhanced Raman scattering (SERS) techniques. In this work, a SERS-active substrate (Si/Au@Ag/ZIF-67) was fabricated with a metal-organic framework (ZIF-67) on a plasmonic surface (Si/Au@Ag) via self-assembly. The as-prepared material combined the properties of the abundant hotspots of the Au@Ag nanoparticles and the excellent adsorption performance of ZIF-67 for organic molecules. The synergy leads to high sensitivity of the composite substrate with a low detection limit for 4-aminothiophenol (a typical Raman reporter molecule) down to 2.0 × 10-9 M and the analytical enhancement factor (AEF) of the SERS substrate is 3.4 × 106. Moreover, the substrates exhibited good repeatability, high reproducibility, and reliable stability due to the MOF coating. The SERS signal was stable after 60 days of storage at room temperature. Ultimately, the optimal Si/Au@Ag/ZIF-67 was applied as a SERS sensor to analyze thiram, and the results showed a linear concentration range from 10-7 to 10-5 M with good linearity (R2 = 0.9934). The recoveries of thiram in spiked apple juice were in the range of 95.7-102.3%, with relative standard deviations less than 4.3%. These results predict that the proposed SERS substrates may hold great potential for the detection of environmental and food pollution in practical applications.