Elucidation of pH impacts on monosubstituted benzene derivatives using normal Raman and surface-enhanced Raman scattering

Raman spectral vibrational frequencies are used to probe the local chemical environment surrounding molecules in solution and adsorbed to gold nanostars. Herein, the impacts of functional group protonation on monosubstituted benzene derivatives with amine, carboxylic acid, or hydroxide are evaluated. Changes in binding affinity and orientation are apparent by evaluating systematic variations in vibrational frequencies. Notably, the electron donating abilities of these functional groups influence the vibrational frequency of the ring breathing mode, thus leading to improved spectral interpretation. Furthermore, gold nanostars are used to investigate the impact of molecular protonation on the adsorption of benzoic acid/benzoate to gold. The changes in molecular protonation are measured using zeta potential and the surface-sensitive technique, surface-enhanced Raman scattering. These methods reveal that pH variations induce carboxylate protonation and electron redistribution that weaken molecular affinity, thereby causing the molecule to adopt a perpendicular to parallel orientation with respect to the nanostar surface. Functional group identity influences the ring breathing mode frequency as a function of changes in electron donation from the functional group to the ring in solution as well as molecular affinity to and orientation on gold. This exploitation of vibrational frequencies facilitates the elucidation of molecule behavior in complex systems.

Damping resonance and refractive index effect on the layer-by-layer sputtering of Ag and Al2O3 on the polystyrene template

In this study, an ordered metal oxide-metal composite system was designed. By changing the thickness of film of Ag/Al₂O₃ nanoparticles (NPs), the red and blue shifts of local surface plasma resonance (LSPR) were realized in the proposed system and discussed by damping resonance theory and Mie's scattering theory to demonstrating the relationship between wavelength (λ) and particle diameter (D). With the increasing of sputtering time of Ag, the SPR of Ag was red shifted under the influence of damped vibration, obtaining that square of wavelength (λ²) is proportional to D. The surface plasma resonance (SPR) of Ag/Al₂O₃ showed an obvious blue shift, and then red shift suddenly, which is affected by the competition between damping resonance and refractive index. When the blue shift occurs, the change of wavelength (∆λ) is exponentially related to the diameter (D). The modulation of LSPR of the proposed composite nano-metal materials will have a potential application in SPR sensor and surface enhanced Raman scattering (SERS).

Preparation of Selective and Reproducible SERS Sensors of Hg2+ Ions via a Sunlight-Induced Thiol⁻Yne Reaction on Gold Gratings

In this contribution, we propose a novel functional surface-enhanced Raman spectroscopy (SERS) platform for the detection of one of the most hazardous heavy metal ions, Hg²⁺. The design of the proposed sensor is based on the combination of surface plasmon-polariton (SPP) supporting gold grating with the high homogeneity of the response and enhancement and mercaptosuccinic acid (MSA) based specific recognition layer. For the first time, diazonium grafted 4-ethynylphenyl groups have undergone the sunlight-induced thiol–yne reaction with MSA in the presence of Eosine Y. The developed SERS platform provides an extremely sensitive, selective, and convenient analytical procedure to detect mercury ions with limit of detection (LOD) as low as 10⁻¹⁰ M (0.027 µg/L) with excellent selectivity over other metals. The developed SERS sensor is compatible with a portable SERS spectrophotometer and does not require the expensive equipment for statistical methods of analysis.

Facile detection of glucose in human serum employing silver-ion-guided surface-enhanced Raman spectroscopy signal amplification

A facile surface-enhanced Raman scattering (SERS) sensor based on a silver-ion-mediated amplification effect was designed for the determination of glucose concentration. In this approach, 4-aminothiophenol (4-ATP) molecules assembled on the surface of a gold wafer (Au wafer@4-ATP) act not only as Raman tags but also as linkage agents. Silver nanoparticles marked with cysteamine (AgNP@cys) were used as the SERS enhancement components because they could be bound to the Au wafer@4-ATP in the presence of silver ions through the formation of N → Ag⁺ ← N coordination compounds. Here, the Ag⁺ ions were obtained by using glucose oxidase to catalyze the oxidation of glucose, producing hydrogen peroxide (H₂O₂) to etch the AgNPs. Therefore, we recorded the SERS intensity of 4-ATP to determine the concentration of glucose in a phosphate buffer as low as 0.1 mM and further achieved a lowest detection of 0.5 mM glucose in human serum. These results show that the proposed approach has strong potential for practical applications.

Glucose Sensing Using Surface-Enhanced Raman-Mode Constraining

Diabetes mellitus is a chronic disease, and its management focuses on monitoring and lowering a patient's glucose level to prevent further complications. By tracking the glucose-induced shift in the surface-enhanced Raman-scattering (SERS) emission of mercaptophenylboronic acid (MPBA), we have demonstrated fast and continuous glucose sensing in the physiologically relevant range from 0.1 to 30 mM and verified the underlying mechanism using numerical simulations. Bonding of glucose to MPBA suppresses the "breathing" mode of MPBA at 1071 cm^-1 and energizes the constrained-bending mode at 1084 cm^-1, causing the dominant peak to shift from 1071 to 1084 cm^-1. MPBA-glucose bonding is also reversible, allowing continuous tracking of ambient glucose concentrations, and the MPBA-coated substrates showed very stable performance over a 30 day period, making the approach promising for long-term continuous glucose monitoring. Using Raman-mode-constrained, miniaturized SERS implants, we also successfully demonstrated intraocular glucose measurements in six ex vivo rabbit eyes within ±0.5 mM of readings obtained using a commercial glucose sensor.

Sensitive Hg2+ Sensing via Quenching the Fluorescence of the Complex between Polythymine and 5,10,15,20-tetrakis(N-methyl-4-pyridyl) Porphyrin (TMPyP)

The interaction between polythymine (dTn) and 5,10,15,20-tetrakis(N-methyl-4-pyridyl) porphyrin (TMPyP) was systematically studied using various techniques. dTn remarkably enhanced the fluorescence intensity of TMPyP as compared to other oligonucleotides. The enhanced fluorescence intensity and the shift of the emission peaks were ascribed to the formation of a π-π complex between TMPyP and dTn. And the quenching of the dTn-enhanced fluorescence by Hg²⁺ through a synergistic effect occurs due to the heavy atom effect. The binding of Hg²⁺ to TMPyP plays an important role in the Hg-TMPyP-dT₃₀ ternary complex formation. A TMPyP-dT₃₀-based Hg²⁺ sensor was developed with a dynamic range of Hg²⁺ from 5 nM to 100 nM. The detection limit of 1.3 nM was low enough for Hg²⁺ determination. The sensor also exhibited good selectivity against other metal ions. Experiments for tap water and river water demonstrated that the detection method was applicable for Hg²⁺ determination in real samples. The Hg²⁺ sensor based on oligonucleotide dT₃₀-enhanced TMPyP fluorescence was fast and low-cost, presenting a promising platform for practical Hg²⁺ determination.

Highly sensitive determination of iron (III) ion based on phenanthroline probe: Surface-enhanced Raman spectroscopy methods

In this paper, we introduced Raman spectroscopy techniques that were based on the traditional Fe³⁺ determination method with phenanthroline as a probe. Interestingly, surface-enhanced Raman spectroscopy (SERS)-based approach exhibited excellent sensitivities to phenanthroline. Different detection mechanisms were observed for the RR and SERS techniques, in which the RR intensity increased with increasing Fe³⁺ concentration due to the observation of the RR effect of the phenanthroline-Fe²⁺ complex, whereas the SERS intensity increased with decreasing Fe³⁺ concentration due to the observation of the SERS effect of the uncomplexed phenanthroline. More importantly, the determination sensitivity was substantially improved in the presence of a SERS-active substrate, giving a detection limit as low as 0.001μg/mL, which is 20 times lower than the limit of the UV-vis and RR methods. Furthermore, the proposed SERS method was free from other ions interference and can be used quality and sensitivity for the determination of the city tap water.

Design of tunable ultraviolet (UV) absorbance by controlling the AgAl co-sputtering deposition

Changing the structure and composition of a material can alter its properties; hence, the controlled fabrication of metal nanostructures plays a key role in a wide range of applications. In this study, the structure of AgAl ordered arrays fabricated by co-sputtering deposition onto a monolayer colloidal crystal significantly increased its ultraviolet (UV) absorbance owing to a tunable localized surface plasmon resonance (LSPR) effect. By increasing the spacing between two nanospheres and the content of aluminum, absorbance in the UV region could be changed from UVA (320-400nm) to UVC (200-275nm), and the LSPR peak in the visible region gradually shifted to the UV region. This provides the potential for surface-enhanced Raman scattering (SERS) in both the UV and visible regions.

SERS study of surface plasmon resonance induced carrier movement in Au@Cu2O core-shell nanoparticles

A plasmon induced carrier movement enhanced mechanism of surface-enhanced Raman scattering (SERS) was investigated using a charge-transfer (CT) enhancement mechanism. Here, we designed a strategy to study SERS in Au@Cu₂O nanoshell nanoparticles with different shell thicknesses. Among the plasmonically coupled nanostructures, Au spheres with Cu₂O shells have been of special interest due to their ultrastrong electromagnetic fields and controllable carrier transfer properties, which are useful for SERS. Au@Cu₂O nanoshell nanoparticles (NPs) with shell thicknesses of 48-56nm are synthesized that exhibit high SERS activity. This high activity originates from plasmonic-induced carrier transfer from Au@Cu₂O to 4-mercaptobenzoic acid (MBA). The CT transition from the valence band (VB) of Cu₂O to the second excited π-π* transition of MBA, and is of b₂ electronic symmetry, which was enhanced significantly. The Herzberg-Teller selection rules were employed to predict the observed enhanced b₂ symmetry modes. The system constructed in this study combines the long-range electromagnetic effect of Au NPs, localized surface plasmon resonance (LSPR) of the Au@Cu₂O nanoshell, and the CT contribution to assist in understanding the SERS mechanism based on LSPR-induced carrier movement in metal/semiconductor nanocomposites.

Sensitive Detection of Rhodamine B in Condiments Using Surface-Enhanced Resonance Raman Scattering (SERRS) Silver Nanowires as Substrate

In this paper, a facile large-scale preparation of surface-enhanced resonance Raman scattering (SERRS) substrates for the determination of Rhodamine B (RhB) based on silver nanowires (Ag NWs) has been developed. The morphology, structure, and properties of as-prepared Ag NWs are characterized using ultraviolet-visible (UV-Vis) spectroscopy, field emission scanning electron microscopy (FE-SEM), and X-ray diffraction (XRD), respectively. Ag NWs were assembled onto glass slides through a self-assembly method. Moreover, in our experiment, as-prepared Ag NWs@glass were used as a SERRS substrate to detect RhB at the excitation wavelength of 532 nm. Experimental conditions such as pH value and soaking time on SERRS performance were studied and optimized. Under the optimized conditions, the SERRS intensity at 1648 cm^-1 exhibited a linear relationship with the concentration of RhB in the range of 1.0 × 10^-9-1.0 × 10^-5mol L^-1 and detection limit (signal-to-noise ratio [S/N] = 3) is as low as 0.3 nmol L^-1. The corresponding correlation coefficient of the linear equation was 0.996. This method based on Ag NWs@glass for the detection of RhB in three kinds of condiment was investigated. The limits of detection (LODs) for RhB were 0.35 µg/g in chili powder, 0.14 µg/g in chili sauce, and 0.02 µg/g in Chinese prickly ash. The relative standard deviations (RSD) were between 2.18% and 4.56% (n = 3) and recoveries at three levels were in the range of 80.0-98.7% for different spiked food products. Moreover, the results showed that the proposed method was sensitive, convenient, and feasible for the determination of RhB in condiments.

Multiplex Immunochips for High-Accuracy Detection of AFP-L3% Based on Surface-Enhanced Raman Scattering: Implications for Early Liver Cancer Diagnosis

α-Fetoprotein (AFP) is an important tumor biomarker. In particular, the overexpression of AFP-L3 is associated with hepatocellular carcinoma (HCC). Accordingly, several hospitals have begun to employ the ratio of AFP-L3 to the total AFP level (AFP-L3%) as new diagnostic evidence for HCC owing to its high diagnostic accuracy. However, current methods of detection for AFP and AFP-L3 are time-consuming, require multiple samples, and lack in sensitivity and specificity. Herein, we present a novel concept for the early diagnosis of HCC based on the combination of Raman frequency shift and intensity change, and developed surface-enhanced Raman scattering (SERS)-based immunochips via AFP-L3%. In the first step of the study, the frequency shift of 4-mercaptobenzoic acid (MBA) was applied for the quantitative determination of total AFP based on the AFP and anti-AFP interaction on MBA-modified silver chips. 5,5-Dithiobis(succinimidyl-2-nitrobenzoate) (DSNB)-modified immunogold was then incorporated with AFP-L3 antibodies for sandwich immunoreaction on the chips. As a result, we found that a typical Raman band intensity of DSNB presented an exponential linear relationship with the concentration of AFP-L3. Thus, the AFP-L3% can be calculated according to the concentrations of AFP-L3 and total AFP. The most important advantage of the proposed method is the combination of AFP-L3% and frequency shifts of SERS, which exhibits excellent reproducibility and high accuracy, and significantly simplifies the conventional detection procedure of AFP-L3%. Application of the proposed method with the serum of patients with HCC demonstrated its great potential in early liver cancer diagnosis.

Surface-Enhanced Raman Scattering (SERS) Active Gold Nanoparticles Decorated on a Porous Polymer Filter

In this work, we designed a process to assemble gold nanoparticles onto a three-dimensional (3D) polymer surface, which can then be monitored using surface-enhanced Raman scattering (SERS). This work is the first demonstration of the assembly of gold nanoparticles on a filter film and in situ measurement with Raman spectroscopy. Herein, a polyhexamethylene adipamide (Nylon66) film embedded in the organic filter film was used as a template to fabricate a tunable SERS-active substrate. A "hotspot"-rich gold-nanoparticle-decorated polymer substrate for SERS was prepared; this substrate exhibited high sensitivity in trace detection of targets. The study was conducted using 4-mercaptobenzoic acid as a probe molecule with the aim of comparing the scattering efficiency and the homogeneity of the Raman signal on selected substrates. In addition, we used the gold-decorated polymer film to detect a biotin-avidin complex. The most powerful advantage of the proposed microanalytical device is the in situ SERS application. The 3D nanoporous structures described in this work hold strong potential for use in various applications such as environmental monitoring and biomolecule detection.