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Phosphate enrichment of the aquatic system often impacts their aesthetic and ecological values. Hence, phosphate recovery is essential to maintain the quality of aquatic systems and ensure sustainable use of phosphorous. Herein, Mg-modified nanobiochar from spent coffee grounds (Mg/NBC) is proposed as an efficient adsorbent for phosphate recovery, and as a phosphorous-release fertilizer. The nanobiochar was obtained by acid digestion of the biochar resulting from pyrolysis. The effect of parameters viz pH, contact time, the dose of adsorbent, and initial concentration on the adsorption was investigated in batch experiments. Results indicated that phosphate adsorption on Mg/NBC is favored in acidic conditions with maximum adsorption (95%) at pH 1. Mg/NBC showed a 12.84% improvement in the adsorption efficiency than the pristine nanobiochar. Characterizations of the adsorbent before and after adsorption suggested that phosphate is involved in both physisorption and chemisorption. Adsorption of phosphate followed pseudo-second-order kinetics with an equilibrium constant (K2) of 0.029 g/mg.min. The Langmuir adsorption isotherm model is better fitted to the equilibrium data with a maximum adsorption capacity of 100 mg/g suggesting uniform monolayer adsorption. Results of the pot test showed that phosphate-laden Mg/NBC improved the growth of garlic and beans by 10 and 5% respectively. The result highlights the utility of phosphate-laden Mg/NBC as a phosphorous-release fertilizer and illustrates a circular economy approach for spent coffee grounds.

Functionalization of a plasmonic substrate circumvents inadequate analyte localization within the sensing volume, which often threatens the sensitivity of a SERS method. Often nanoplasmonic substrates are functionalized by direct attachment to the host molecule, which affects their SERS activity. Herein, we report 3D-amino-functionalized Ag@SiO2 nanocubes with ultra-thin silica shell for the detection of small molecules in a flexible platform. The amino groups are introduced by silanizing the ultra-thin silica layer using different aminoslanes which improves sensitivity without sacrificing the colloidal and chemical stability of the core Ag nanocubes. Ag-SHINs functionalized with shorter chain length aminosilanes shows better SERS activity. The amino-functionalization effects a 3-fold SERS signal enhancement enabling label-free aspartame detection down to 71 μg mL−1 which is 1.5 times lower LOD than the non-functionalized Ag-SHINs. Analysis of spiked soft drink, as well as a mixture of aspartame and glucose, shows a robust performance signaling the selectivity and sensitivity of the substrate. The improved SERS performance is attributed to the hydrogen bonding induced localization of the analyte within the sensing volume. Added to this, the miniaturized paper platform with its 3D fibrous structure enhances the hotspot density and sample collection efficiency. This substrate can also be extended to other small molecules with the carboxylic group and amino acids as well, replacing the complicated bio-labeled detection schemes.

The translation of Surface Enhanced Raman Spectroscopy (SERS) from the laboratory to real-world applications such as point-of-need detection requires the fabrication of inexpensive yet quality substrates. Due to its flexibility, biodegradability, and 3D cellulosic structure, paper can be an excellent platform for the fabrication of low-cost, flexible SERS substrates that could expand the application of SERS to point-of-need sensing applications. In this mini-review, recent advances in paper-based SERS substrates along with various fabrication methods for plasmonic paper are discussed. The pros and cons of different fabrication methods are evaluated from the viewpoint of mass production. The different approaches to improving the performances of plasmonic paper substrates and additional analytical functionalities obtained from plasmonic paper substrates are discussed. Finally, opportunities and challenges in the realization of a low-cost commercial plasmonic paper substrate are also suggested.

A robust flexible paper-plasmonic hybrid SERS substrate is reported by exploiting the intrinsic properties of filter paper and anisotropic Ag@SiO2 nanocubes. The fibrous structure of paper promotes the assemblage of sharp-edged nanostructures which significantly improved the SERS activity. An inexpensive sensing platform with reasonable distribution and interparticle spacing of nanocubes was obtained by filtering Ag@SiO2 through a miniaturized filter paper. The thin silica shell improved the stability and interparticle spacing of silver nanocubes in the sensing platform, rendering enhanced SERS activity through the plasmon-coupling effect as compared to a conventional rigid substrate. Assessment of the analytical performance of the substrate for melamine quantification showed a good linearity (R2 = 0.9948) up to 1 mg L−1 with a limit of detection of 0.06 mg L−1. The detection limit in liquid milk was down to 0.17 mg L−1, which is below the permissible residue limit signifying adequate sensitivity for real sample analysis with less sample treatment.

The interaction of plasmonic nanoparticles with a dielectric platform gives rise to unique optical behaviors and this can be maneuvered to improve the plasmonic/SERS performances of a substrate. Herein, dielectric modified plasmonic-paper SERS substrate is developed by assembling Ag@SiO2 nanocubes on Fe-TiO2 nanosheets (NS) modified paper. The Fe-TiO2 NS being visible light responsive significantly alters the optical property of the paper and serves as a dielectric underlay for the Ag nanocubes. Hence, the incident light reflected back from the dielectric nanosheets couples with the scattered light from the Ag nanocubes leading to spatially enhanced electromagnetic field improving the SERS enhancement. The prepared dielectric modified plasmonic-paper has an average enhancement factor (EF) of 1.49 × 107 using R6G as a probe molecule. This value is superior to unmodified plasmonic-paper highlighting the coupling effect of the dielectric nanosheets. The substrate shows robust detection performance for thiabendazole and achieves a limit of detection (LOD) of 19 μg/L, which is 4-fold more sensitive than unmodified plasmonic paper. Direct swabbing test of thiabendazole sprayed apple fruit shows a discernible Raman signal down to 15 ppb indicating the utility of the substrate for point-of-need applications in food safety.

Deoxynivalenol (DON) is one of the trichothecene mycotoxin, a frequent contaminant of pig feed. Surface-enhanced Raman spectroscopy (SERS) is a fast and ultrasensitive analytical tool for point-of-need applications to identify molecular fingerprint structures at low concentrations. However, the use of SERS for analyte detection with flexible and robust structures is still challenging. Herein, we have developed core-shell silver nanocubes coated with polydopamine (Ag NCs@PDA) SERS substrate for the quantitative detection of deoxynivalenol in pig feed. The Ag NCs@PDA substrate with ultrathin (1.6 nm) PDA shell thickness enhances the absorption of DON via hydrogen bonding and π–π stacking interactions, as well as improves the stability of the substrate. The results of the SERS showed a high analytical enhancement factor (AEF) of 1.82 × 107 and a detection limit (LOD) as low as femtomolar range (0.82 fM). The LOD of the Ag NCs@PDA substrate for DON detection is 1.8 times lower than the bare Ag NCs. Furthermore, the Ag NCs@PDA substrate is stable which retains 88.24% of the original Raman intensity after storage for three months. The obtained results demonstrate that the Ag NCs@PDA substrates can realize label-free detection of deoxynivalenol mycotoxin with high sensitivity, reproducibility, and stability. Our work proposes a low-cost method for the designing of the SERS sensing device, and has great potential to be applied in food safety, biomedical sciences, and environmental monitoring.

Surface Enhanced Raman Scattering (SERS), benefitted by its fingerprinting ability of molecules can be a reliable option for trace chemical analysis. Since the sensitivity of a SERS, method is highly dependent on the degree, to which the Raman signature is enhanced, fabrication substrates that could adequately amplify the local field through excitation of localized surface plasmon resonances (LSPRs) is critical. Hence, control over the nanostructures morphology and improving their inter-particle distance is important for obtaining improved SERS activity. In addition, flexible platforms like paper offer the means for improving the nanostructures assemblage with ease of fabrication at low cost and improved sample collection efficiency.Here in, inexpensive, flexible paper-plasmonic hybrid SERS substrate is reported by loading Ag@SiO2 nanocubes on a miniaturized filter paper through vacuum filtration. The miniaturized sensing platform owes a reasonable distribution and inter-particle spacing of nanocubes. The fibrous structure of paper promotes the assemblage of sharp-edged nanostructures which significantly improve their distribution and SERS activity.The thin silica shell improved the stability and inter-particle spacing of silver nanocubes in the sensing platform, rendering enhanced SERS activity through plasmon-coupling effect as compared to a conventional rigid substrate. Assessment of analytical performances of the substrate for melamine quantification showed a good linearity (R2 = 0.9947) up to 1 mg/L with a limit of detection 0.06 mg/L. The detection limit in liquid milk was down to 0.17 mg/L, which is below the permissible residue limit signifying adequate sensitivity for real sample analysis with less sample treatment.