A nanoaggregate-on-mirror platform for molecular and biomolecular detection by surface-enhanced Raman spectroscopy

Recent advances in detection techniques and chemometric methods for identifying adulterants in milk and dairy products

Milk adulteration poses a significant threat to food safety, driven by increasing global demand for milk and dairy products, particularly in developing countries. Supply limitations, compounded by challenges like pandemics and livestock diseases such as mastitis, often lead to unethical practices that compromise product quality and consumer health. Adulterants, including preservatives, non-milk fats, thickening agents, nitrogen-based compounds, and surfactants, are intentionally added to mimic natural properties or enhance the solid-not-fat (SNF) content of milk. Advanced detection techniques, such as spectroscopy, chromatography, biosensors (protein and DNA-based), and electromigration methods like SDS-PAGE, have emerged as critical tools for identifying these adulterants. Innovative approaches, including electrical sensors like electronic noses and tongues, further enhance detection accuracy. This study highlights recent advancements in instrumental techniques alongside the pivotal role of chemometric methods in analyzing complex datasets, improving precision, and ensuring reliable detection, offering insights into current progress, challenges, and opportunities in safeguarding milk integrity.

Ratio type nanoprobe with boric acid as recognition unit for imaging intracellular H2O2 with SERS

Fluctuations of intracellular H2O2 level are intricately linked to diverse biological processes, e.g., cellular proliferation, biosynthesis, and metabolic activities. The dynamics of intracellular H2O2 levels holds immense significance as it represents a pivotal metabolite within the realm of free radicals. This study describes a novel internal standard assisted surface enhanced Raman scattering (SERS) nanoprobe based on H2O2 boronic acid oxide group for precise detection of H2O2 levels in vitro and within living cells. The nanoprobe consists of a core molecule shell Au nanostar (Au@MPBN@Au NPs) embedded with internal marker (4-mercaptobenzonitrile, 4-MPBN) as a high plasma active SERS substrate and a 4-mercaptophenylboronic acid (4-MPBA) molecule fixed on its surface as H2O2 recognition unit. The presence of H2O2 leads to a decrease of the band intensity for B-O group at 998 cm-1, while the absorption for the internal standard molecule 4-MPBN at 2223 cm-1 remains unaffected by the variation of external environment. H2O2 quantification may be achieved by analyzing the ratio of band intensities at I2223 cm-1/I998 cm-1, with a linear relationship between I2223cm-1/I998cm-1 and H2O2 concentration within the range of 5-100 μM, along with a limit of detection (LOD) of 1.97 μM. Meanwhile, due to the presence of internal marker molecules, measurement errors caused by environmental or instrumental fluctuations can be calibrated in real-time. The AuNPs/4-MPBN/4-MPBA nanoprobe provides an accurate tool for dynamic monitoring and quantitative characterization of intracellular H2O2 content during cell apoptosis or other cell growth processes.

Plasmon-driven substitution of 4-mercaptophenylboronic acid to 4-nitrothiophenol monitored by surface-enhanced Raman spectroscopy

Plasmon-driven reactions on plasmonic nanoparticles (NPs) occur under significantly different conditions from those of classical organic synthesis and provide a promising pathway for enhancing the efficiency of various chemical processes. However, these reactions can also have undesirable effects, such as 4-mercaptophenylboronic acid (MPBA) deboronation. MPBA chemisorbs well to Ag NPs through its thiol group and can subsequently bind to diols, enabling the detection of various biological structures by surface-enhanced Raman scattering (SERS), but not upon its deboronation. To avoid this reaction, we investigated the experimental conditions of MPBA deboronation on Ag NPs by SERS. Our results showed that the level of deboronation strongly depends on both the morphology of the system and the excitation laser wavelength and power. In addition, we detected not only the expected products, namely thiophenol and biphenyl-4,4-dithiol, but also 4-nitrothiophenol (NTP). The crucial reagent for NTP formation was an oxidation product of hydroxylamine hydrochloride, the reduction agent used in Ag NP synthesis. Ultimately, this reaction was replicated by adding NaNO2 to the system, and its progress was monitored as a function of the laser power, thereby identifying a new reaction of plasmon-driven -B(OH)2 substitution for -NO2.

Glucose detection through surface-enhanced Raman spectroscopy: A review

Glucose detection is of vital importance to diabetes diagnosis and treatment. Optical approaches in glucose sensing have received much attention in recent years due to the relatively low cost, portable, and mini-invasive or non-invasive potentials. Surface enhanced Raman spectroscopy (SERS) endows the benefits of extremely high sensitivity because of enhanced signals and specificity due to the fingerprint of molecules of interest. However, the direct detection of glucose through SERS was challenging because of poor adsorption of glucose on bare metals and low cross section of glucose. In order to address these challenges, several approaches were proposed and utilized for glucose detection through SERS. This review article mainly focuses on the development of surface enhanced Raman scattering based glucose sensors in recent 10 years. The sensing mechanisms, rational design and sensing properties to glucose are reviewed. Two strategies are summarized as intrinsic sensing and extrinsic sensing. Four general categories for glucose sensing through SERS are discussed including SERS active platform, partition layer functionalized surface, boronic acid based sensors, and enzymatic reaction based biosensors. Finally, the challenges and outlook for SERS based glucose sensors are also presented.

From single cells to complex tissues in applications of surface-enhanced Raman scattering

This tutorial review explores how three of the most common methods for introducing nanoparticles to single cells for surface-enhanced Raman scattering measurements can be adapted for experiments with complex tissues.

Aggregation of Gold Nanoparticles Caused in Two Different Ways Involved in 4-Mercaptophenylboronic Acidand Hydrogen Peroxide

The difference in gold nanoparticle (AuNPs) aggregation caused by different mixing orders of AuNPs, 4-mercaptophenylboronic acid (4-MPBA), and hydrogen peroxide (H₂O₂) has been scarcely reported. We have found that the color change of a ((4-MPBA + AuNPs) + H₂O₂) mixture caused by H₂O₂ is more sensitive than that of a ((4-MPBA + H₂O₂) + AuNPs) mixture. For the former mixture, the color changes obviously with H₂O₂ concentrations in the range of 0~0.025%. However, for the latter mixture, the corresponding H₂O₂ concentration is in the range of 0~1.93%. The mechanisms on the color change originating from the aggregation of AuNPs occurring in the two mixtures were investigated in detail. For the ((4-MPBA + H₂O₂) + AuNPs) mixture, free 4-MPBA is oxidized by H₂O₂ to form bis(4-hydroxyphenyl) disulfide (BHPD) and peroxoboric acid. However, for the ((4-MPBA+AuNPs) + H₂O₂) mixture, immobilized 4-MPBA is oxidized by H₂O₂ to form 4-hydroxythiophenol (4-HTP) and boric acid. The decrease in charge on the surface of AuNPs caused by BHPD, which has alarger steric hindrance, is poorer than that caused by -4-HTP, and this is mainly responsible for the difference in the aggregation of AuNPs in the two mixtures. The formation of boric acid and peroxoboric acid in the reaction between 4-MPBA and H₂O₂ can alter the pH of the medium, and the effect of the pH change on the aggregation of AuNPs should not be ignored. These findings not only offer a new strategy in colorimetric assays to expand the detection range of hydrogen peroxide concentrations but also assist in deepening the understanding of the aggregation of citrate-capped AuNPs involved in 4-MPBA and H₂O₂, as well as in developing other probes.

Fabrication and Characterization of a Highly-Sensitive Surface-Enhanced Raman Scattering Nanosensor for Detecting Glucose in Urine

Herein we utilized coordination interactions to prepare a novel core-shell plasmonic nanosensor for the detection of glucose. Specifically, Au nanoparticles (NPs) were strongly linked with Ag+ ions to form a sacrificial Ag shell by using 4-aminothiophenol (4-PATP) as a mediator, which served as an internal standard to decrease the influence of the surrounding on the detection. The resultant Au-PATP-Ag core-shell systems were characterized by UV-vis spectroscopy, transmission electron microscopy, and surface-enhanced Raman scattering (SERS) techniques. Experiments performed with R6G (rhodamine 6G) and CV (crystal violet) as Raman reporters demonstrated that the Au@Ag nanostructure amplified SERS signals obviously. Subsequently, the Au@Ag NPs were decorated with 4-mercaptophenylboronic acid (4-MPBA) to specifically recognize glucose by esterification, and a detection limit as low as 10⁻⁴ M was achieved. Notably, an enhanced linearity for the quantitative detection of glucose (R² = 0.995) was obtained after the normalization of the spectral peaks using 4-PATP as the internal standard. Finally, the practical applicability of the developed sensing platform was demonstrated by the detection of glucose in urine with acceptable specificity.

Multi-functional, thiophenol-based surface chemistry for surface-enhanced Raman spectroscopy

This article highlights the recent advances of thiophenol-based surface chemistry for the applications in surface-enhanced Raman spectroscopy (SERS).