Polyethylenimine-capped Ag nanoparticle film as a platform for detecting charged dye molecules by surface-enhanced Raman scattering and metal-enhanced fluorescence

Unveiling Electrostatic Interaction Modulated Sensitive and Selective SERS Detection for Negatively Charged Molecules

Strengthening the interaction between the target and SERS substrate is crucial for sensitive SERS detection; we thereby explored the molecular structure-dependent SERS sensitivity for negatively charged targets on the positively charged SERS substrate. Both experimental and theoretical studies confirm that the SERS sensitivity is determined by the electrostatic interaction between the target and linker. This interaction is not only manipulated by the protonation capacity of the linker and its surface adsorption configuration and geometry but also significantly determined by the target's structure, encompassing electronegativity and the number of interaction sites. The optimized interaction leads to a marked improvement in detection sensitivity of up to 1-3 orders of magnitude. The interaction mechanism revealed in this work not only provides theoretical guidance and technical support for electrostatically driven SERS detection but also offers a conceptual framework that can be extended to various SERS detections based on diverse surface forces.

Positively Charged Silver and Gold Nanoparticles with Controllable Size Distribution for SERS Detection of Negatively Charged Molecules

Surface-enhanced Raman spectroscopy (SERS) has been demonstrated as an ultrasensitive tool for various molecules. However, for the negatively charged molecules, the widely used SERS substrate [negatively charged Ag and Au nanoparticles (Ag or Au NPs (-)] showed either low sensitivity or poor stability. The best solution is to synthesize positively charged silver or gold nanoparticles [Ag or Au NPs (+)] with high stability and excellent SERS performance, which are currently unavailable. To this end, we revitalized the strategy of "charge reversal and seed growth". By selection of ascorbic acid as the reductant and surfactant, the surface charge of Ag or Au NP (-) seeds is adjusted to a balanced state, where the surface charge is negative enough to satisfy the stabilization of the NPs (-) but does not hinder the subsequent charge reversal. By optimization of the chain length and electric charge of polyamine molecules, the highly stable and size-controllable uniform Ag NPs (+) and Au NPs (+) were seed-growth synthesized with high reproducibility. More importantly, the SERS performance of both Ag NPs (+) and Au NPs (+) achieved the trace detection of negatively charged molecules at the level of 1 μg/L, demonstrating an improved SERS sensitivity of up to 3 orders of magnitude compared to the previously reported sensitivity. Promisingly, the introduction of polyamine-capped Ag NPs (+) and Au NPs (+) as SERS substrates with high stability (1 year shelf life) will significantly broaden the application of SERS.

Physical and physiological characteristics, as well as miRNA concentrations, are affected by the storage time of tomatoes

This study aims to investigate the potential of miRNA measurements in indicating tomato quality during transportation and storage. The impact of storage temperature, duration, and mechanical loading on tomato senescence, carotenoid content, total soluble solids, fruit firmness, and relevant miRNA concentrations were examined. Significant two-way or three-way interactions were observed between storage conditions and physical/physiological characteristics (excluding carotenoids). Remarkably, significant three-way interactions were found between storage conditions and miRNA concentrations. Strong correlations were observed between the physiological characteristics of the tomatoes and their miRNA concentrations. These findings suggest that measuring miRNAs could serve as a convenient and portable method for evaluating postharvest fruit quality, reducing reliance on labor-intensive laboratory techniques.

Positively-charged plasmonic nanostructures for SERS sensing applications

Surface-enhanced Raman (SERS) spectroscopy has been establishing itself as an ultrasensitive analytical technique with a cross-disciplinary range of applications, which scientific growth is triggered by the continuous improvement in the design of advanced plasmonic materials with enhanced multifunctional abilities and tailorable surface chemistry. In this regard, conventional synthetic procedures yield negatively-charged plasmonic materials which can hamper the adhesion of negatively-charged species. To tackle this issue, metallic surfaces have been modified via diverse procedures with a broad array of surface ligands to impart positive charges. Cationic amines have been preferred because of their ability to retain a positive zeta potential even at alkaline pH as well as due to their wide accessibility in terms of structural features and cost. In this review, we will describe and discuss the different approaches for generating positively-charged plasmonic platforms and their applications in SERS sensing.

Preparation and application of silver nanoparticle-functionalized magnetic graphene oxide nanocomposites

Although selective enrichment of glycopeptides from complex biological samples is indispensable for mass spectrometry (MS)-based glycoproteomics, it still remains a great challenge due to the low abundance of glycoproteins and suppression of non-glycopeptides. In this study, silver nanoparticle-functionalized magnetic graphene oxide nanocomposites (GO/Fe₃O₄/PEI/Ag) were synthesized. Silver nanoparticles were generated in situ on the surface of magnetic graphene oxide using polyethylenimine as a reducing and stabilizing agent. The resulting material was used as an adsorbent for selective enrichment of glycopeptides. GO/Fe₃O₄/PEI/Ag nanocomposites offered excellent enrichment ability, which was attributed to the synergistic effect of polyethylenimine and silver nanoparticles. The nanocomposites showed superior specificity for glycopeptides even when non-glycopeptides were 100 times more concentrated than glycopeptides. The nanocomposites displayed advantages including rapid adsorption (1 min), low detection limit (25 fmol), repeatability (6 times), and high recovery (77.8%). Using these nanocomposites, 91 different glycoproteins and 136 N-linked glycopeptides were identified from among 20 μg tryptic human serum proteins and this demonstrated the superior performance of the nanocomposites for glycopeptides enrichment.

A comparison of SERS and MEF of rhodamine 6G on a gold substrate

Multilayer films of rhodamine 6G can act as its own dielectric layer on gold surfaces to enhance emission spectra.

Investigation of the fluorescence quenching behavior of PEI-doped silica nanoparticles and its applications

A simple and feasible method for overcoming the fluorescence quenching effect of PEI on fluorophores (eosin Y was chosen as the model dye) was designed for the first time.

Quantitative understanding of the optical properties of a single, complex-shaped gold nanoparticle from experiment and theory

We report on a combined study of Rayleigh and Raman scattering spectroscopy, 3D electron tomography, and discrete dipole approximation (DDA) calculations of a single, complex-shaped gold nanoparticle (NP). Using the exact reconstructed 3D morphology of the NP as input for the DDA calculations, the experimental results can be reproduced with unprecedented precision and detail. We find that not only the exact NP morphology but also the surroundings including the points of contact with the substrate are of crucial importance for a correct prediction of the NP optical properties. The achieved accuracy of the calculations allows determining how many of the adsorbed molecules have a major contribution to the Raman signal, a fact that has important implications for analyzing experiments and designing sensing applications.

Facile detection of polycyclic aromatic hydrocarbons by a surface-enhanced Raman scattering sensor based on the Au coffee ring effect

Surface-enhanced Raman scattering (SERS) analysis of environmental hydrophobic pollutants without chemical functionalization of a bare nanoparticle (NP) substrate presents a challenge. The motivation for our study is to develop a highly reproducible and robust portable SERS sensor for detection and identification of polycyclic aromatic hydrocarbons (PAHs) using bare Au NPs. Our hypothesis is that the coffee ring effect could separate PAHs from the bulk solution and concentrate them on the closely packed Au NP ring, consequently enhancing their Raman scattering. This premise was confirmed with the commonly used citrate-reduced Au NPs in 20 nm, having no structural uniqueness. Because of the coffee ring effect, however, closely packed but not aggregated Au NP arrays were formed and, consequently, facilitated the separation and concentration of hydrophobic PAHs. As a result, a prominent SERS enhancement can be obtained on the ring because of the electromagnetic mechanism. A mixture of six PAHs with different numbers of benzene rings, namely, naphthalene, anthracene, pyrene, benzo[a]pyrene, benzo[g,h,i]perylene, and indeno[1,2,3-cd]pyrene, could be readily identified in river water. This portable SERS sensor based on the coffee ring effect provides a robust and versatile approach in PAH detection without the need for stringent structural requirements for Au NPs.

Simultaneous SERS and surface-enhanced fluorescence from dye-embedded metal core–shell nanoparticles

We demonstrate a methodology to prepare Au-core–Ag-shell nanoparticles displaying both SERS and surface-enhanced fluorescence (SEF) activities simultaneously by embedding dye molecules between the core and the shell.