Ultrahigh Enhancement Factor Research Articles

Self-assembled C-Ag hybrid nanoparticle on nanoporous GaN enabled ultra-high enhancement factor SERS sensor for sensitive thiram detection

Considering pesticide residues cause significant harm to public health and the environment, developing a simple, sensitive, and reliable approach to pesticide residue detection to address this issue is necessary. In this study, an ultrasensitive and reliable surface-enhanced Raman scattering (SERS) sensor was developed using cetylpyridinium chloride as a protecting and reducing agent for the in situ synthesis and self-assembly of C-Ag nanoparticles on nanoporous GaN for the quantitative detection of thiram. A systematic investigation of the performance of the SERS sensor revealed that the SERS sensor delivered a limit of detection (LOD) of 10−14 M and an enhancement factor of up to 1.80 × 1011 with reasonable uniformity and reproducibility, with the stability of the SERS sensor demonstrated via long-term storage for up to 22 weeks in air. The enhancement mechanism of the SERS sensor was verified using a finite-difference time-domain simulation. The SERS sensor successfully detected thiram in real samples with an LOD of 10−10 M. Hence, this study provides an effective platform for monitoring food safety and the environment.

Confined-Enhanced Raman Spectroscopy.

In 1997, the discovery of single molecule-surface enhanced Raman spectroscopy (SM-SERS) rekindled broad interests owing to its ultrahigh enhancement factor up to the 1014-1015 level. However, regretfully, the advantage of SM-SERS with an ultralow detection limit has not yet been fully utilized in commercialized applications. Here, we report a strategy, which we name confined-enhanced Raman spectroscopy, in which the overall Raman properties can be remarkably improved with in situ-formed active nanoshell on the surface of silver or gold nanoparticles. The nanoshell can confine and anchor molecules onto the surface of plasmonic nanoparticles and avoid desorption from hot spots so that the "on and off" blinking effect can be eliminated. It is the first time the single-molecule detection of analytes with super sensitivity, high stability, and reproducibility based on gold nanoparticles has been realized. In addition, this strategy is suitable for SERS detection in diverse molecule systems, including biomedical diagnosis, catalytic reaction, etc.

Fano-resonant propagating plexcitons and Rabi-splitting local plexcitons of bilayer borophene in TERS

Optical nanocavity provides an opportunity to deeply study the light–matter interaction with notable findings such as Rabi splitting in strong coupling and Fano resonance in weak coupling. Here, we theocratically explore the plexcitons of a bilayer (BL) borophene synthesized on an Ag (1 1 1) film in a tip-enhanced Raman scattering (TERS) system, where the BL borophene is located in the nanocavity between the tip and substrate, stimulated by recent experimental synthesis [Liu et al., Nat. Mater. 21, 35 (2022)]. In the strong-coupling region, the negative real part of the dielectric function of the BL borophene manifests; the BL borophene is of plasmonic properties resulting in Rabi splitting of plexcitons with 310 meV. In the weak-coupling region, the spectra show typical asymmetry with a sharp change between a dip and a peak (Fano resonance). A balanced gain and loss facilitates single-mode lasing in the parity-time symmetry-broken regime, where single-mode lasing with a very narrow half-width is of ultrahigh enhancement factor up to 108. Fano-resonant propagating plexcitons are observed in the dip of Fano resonance, which is extremely sensitive to the excitation wavelength. Our results not only deepen the physical understanding of the plasmon–exciton coupling interaction in the TERS system but also provide a way to manipulate the light–matter interaction in the TERS system.

Stable and highly efficient Co–Bi nanoalloy decorated on reduced graphene oxide (Co–Bi@rGO) anode for formaldehyde and urea oxidation reactions

Extremely energetic and low-cost novel nanoelectrodes for fuel cell reactions are always important for examining more elaborative electrochemical studies at interfacial electron transfer reactions. Herein, this work demonstrated a bifunctional electrocatalyst with its oxophilic character and promoting the role of p-electrons of Bi to be supported with Co (nanoalloy) and further decoration on reduced graphene oxide i.e. Co–Bi@rGO presenting as an auspicious electrochemical activity towards formaldehyde and urea oxidation reactions. Synthesized Co–Bi@rGO composite have been well characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), BET surface area measurement, and X-ray photoelectron spectroscopy (XPS) studies, etc. Further, the electrochemical evaluation shows exceptionally higher activity towards formaldehyde and urea oxidation reactions having onset potential of 0.32 V, 0.35 V vs. SCE respectively. Interestingly, ultra-high enhancement factor for Co–Bi@rGO could be initiated from the cooperative role of Bi with Co centres, assisting for formaldehyde and urea oxidation reactions. Electrochemical impedance spectroscopy (EIS) and chronoamperometry (i-t) studies using Co–Bi@rGO nanocomposite shows excellent long-term current/potential stability with lower charge transfer resistance towards the formaldehyde and urea molecules. This work deals with practical information for the build of a stable and proficient electrocatalyst for direct formaldehyde and urea fuel cells and also will extendable towards the industrial waste water treatment.

Surface-enhanced Raman spectroscopy for environmental monitoring using gold clusters anchored on reduced graphene oxide

Surface-enhanced Raman spectroscopy is a strong and sensitive analysis tool that can realize single-molecule level detection and provide the fingerprint information of molecules, which has been widely applied in analysing chemistry and biomolecules and monitoring environment. However, it is still a challenge to design and prepare SERS substrates with high enhancement factor, simple synthesis, stability and reproducibility. Here, we synthesized gold clusters anchored on reduced graphene oxide (Au clusters@rGO) using co-reduction method to achieve high SERS enhancement. The substrate of gold clusters anchored on reduced graphene oxide combines the chemical enhancement of reduced graphene oxide and the electromagnetic enhancement of gold clusters, leading to an ultrahigh enhancement factor of 3.5 × 107. The efficient SERS was ascribed to the high localized surface plasmon resonance (LSPR) of aggregations of gold clusters, the synergistic effect of gold clusters and reduced graphene oxide, and the charge transfer between graphene and the molecules. This research will provide an invaluable strategy to design and prepare superior-property SERS substrates.

Single-Molecule Surface-Enhanced Raman Spectroscopy.

Single-molecule surface-enhanced Raman spectroscopy (SM-SERS) has the potential to detect single molecules in a non-invasive, label-free manner with high-throughput. SM-SERS can detect chemical information of single molecules without statistical averaging and has wide application in chemical analysis, nanoelectronics, biochemical sensing, etc. Recently, a series of unprecedented advances have been realized in science and application by SM-SERS, which has attracted the interest of various fields. In this review, we first elucidate the key concepts of SM-SERS, including enhancement factor (EF), spectral fluctuation, and experimental evidence of single-molecule events. Next, we systematically discuss advanced implementations of SM-SERS, including substrates with ultra-high EF and reproducibility, strategies to improve the probability of molecules being localized in hotspots, and nonmetallic and hybrid substrates. Then, several examples for the application of SM-SERS are proposed, including catalysis, nanoelectronics, and sensing. Finally, we summarize the challenges and future of SM-SERS. We hope this literature review will inspire the interest of researchers in more fields.

Tuning the aggregation of silver nanoparticles with carbon dots for the surface-enhanced Raman scattering application

Single-layer carbon dots (CDs) are used as capping agents to synthesize silver nanoparticles (AgNPs). The obtained nanohybrids (AgNPs/CDs) can be self-assembled into nanostructures like nanochains (1D-AgNPs/CDs), nanoflats (2D-AgNPs/CDs) or nanobodys (3D-AgNPs/CDs) by simply tuning the amount of added CDs. It is found that CDs play a key role in controlling the aggregation AgNPs/CDs. The formation mechanisms of the AgNPs/CDs aggregates have been discussed. On the basis, the effects of aggregation dimension on the surface plasmon absorption and surface enhanced Raman spectroscopy (SERS) activity of AgNPs/CDs are investigated and discussed. It is found that the aggregation of AgNPs/CDs creates strong localized surface plasmon resonance. Furthermore, the aggregated AgNPs/CDs of different dimension have similar absorption intensity in the range from 500 to 800 nm, which is most commonly used in the surface enhanced Raman spectroscopy (SERS) measurement. The outstanding local electromagnetic field of the aggregated AgNPs and the enrichment effect of the CDs towards the analytes make all the obtained materials to be SERS substrates. In particular, 2D-AgNPs/CDs exhibit an ultrahigh enhancement factor of 4.02 × 1014 and a good reproducibility during the SERS test, using crystal violet as the model target molecule. The applicability of 2D-AgNPs/CDs in SERS detection is further confirmed by the measurement of trace thiram residual in apple.

Hybridizing Carbon-Based Dot-Capped Manganese Dioxide Nanosheets and Gold Nanoparticles as a Highly Sensitive Surface-Enhanced Raman Scattering Substrate.

Surface-enhanced Raman Scattering (SERS) is a sensitive and nondestructive technique that provides fingerprint structural information of molecules. Designing and constructing sensitive and stable SERS substrates is of great significance for the application of the technique. In this study, single-layer carbon-based dots (CDs) are used as capping agents to synthesize gold nanoparticles (AuNPs/CDs) and manganese dioxide nanosheets (MnO2/CDs), which are then hybridized through a simple cocentrifugation method. After the hybridization, the monodispersive AuNPs/CDs aggregate obviously into some clusters exhibiting strong SERS activity due to the electromagnetic "hot spots" effect, and the MnO2/CDs also show outstanding SERS activity due to the charge-transfer resonance effect. The obtained nanohybrids (MnO2/CDs/AuNPs) with robust chemical stability combine well with the electromagnetic enhancement of AuNPs/CDs and chemical enhancement of MnO2/CDs, leading to an ultrahigh enhancement factor of 3.9 × 108. Based on the novel SERS substrate, a sensitive and rapid sensing system for the detection of malachite green is developed, with a low detection limit of 1 × 10-9 M. This work provides a valuable model for designing and fabricating high-performance SERS substrates.

Controlled synthesis of a PS/Au/ZIF-8 hybrid structure as a SERS substrate for ultrasensitive detection

A PS/Au/ZIF-8 hybrid was successfully prepared and used as a SERS active substrate. Meanwhile, the hybrid SERS substrate exhibited good SERS reproducibility, and we successfully obtained an ultrahigh enhancement factor of 1.67 × 106.

Carbon based dots capped tin oxide nanosheets hybridizing with silver nanoparticles for ultra-sensitive surface enhanced raman scattering substrate

Carbon based dots capped tin oxide nanosheets (SnO2/CDs) were synthesized by liquid exfoliation of tin oxide (SnO2) nanoparticles in the presence of single-layer carbon based (CDs). The obtained SnO2/CDs have lateral sizes of about 20–50 nm and heights of about 3–4 nm. A thin layer of CDs of about 1–2 nm in thickness are presence on the surface of the SnO2 nanosheets. The SnO2/CDs present excellent surface enhanced Raman spectroscopy (SERS) activity due to the chemical enhancement (CM) led by the charge transfer between SnO2/CDs and the target molecules. The enhancement factor (EF) of the SnO2/CDs is 1.42 × 105, taking rhodamine 6G as a model target molecule. The SnO2/CDs are further hybridized with silver nanoparticles (AgNPs) using an in situ synthesis method. The obtained homogeneous nano-hybrids (SnO2/CDs@AgNPs) have a lot of nanogaps. The nanogaps among the AgNPs/CDs create strong electromagnetic enhancement (EM) due to the “hot spots” effect. The nanogaps among the SnO2/CDs and AgNPs/CDs allow the target molecules to be embedded, and thus gained both the CM effect of SnO2/CDs and the EM effect of AgNPs/CDs. As a result, the as-prepared SnO2/CDs@AgNPs exhibits excellent SERS activities, with an ultrahigh enhancement factor of 3.27 × 1011.

Increased O 2p State Density Enabling Significant Photoinduced Charge Transfer for Surface-Enhanced Raman Scattering of Amorphous Zn(OH)2.

Enriching the electronic density of states (DOS) of semiconductors is the key to promoting charge transfer (CT) and achieving a large surface-enhanced Raman scattering (SERS) enhancement. Metal hydroxide semiconductors are anticipated to exhibit DOS that are higher than those of metal oxide because of their abundant O atoms; however, their SERS activity has not been verified. Here, combining density functional theory and experiments, we report a SERS sensitivity of amorphous Zn(OH)2 [a-Zn(OH)2] that is much higher than that of amorphous ZnO (a-ZnO), ascribed to the abundant O atoms and hence enriched O 2p state density near the Fermi level in a-Zn(OH)2, which gives rise to higher CT probabilities. Moreover, we find a-Zn(OH)2 exhibits significant advantages in energy-level matching over a-ZnO for efficient photoinduced CT via strong vibronic coupling, ascribed to the upshifted valence band maximum and the narrower band gap of a-Zn(OH)2. Via the synthesis of a-Zn(OH)2 nanocages, an ultrahigh enhancement factor of 1.29 × 106 is obtained in semiconductor-based SERS.

Two-Dimensional Amorphous TiO2 Nanosheets Enabling High-Efficiency Photoinduced Charge Transfer for Excellent SERS Activity.

Substrate-molecule vibronic coupling enhancement, especially the efficient photoinduced charge transfer (PICT), is pivotal to the performance of nonmetal surface-enhanced Raman scattering (SERS) technology. Here, through developing novel two-dimensional (2D) amorphous TiO2 nanosheets (a-TiO2 NSs), we successfully obtained an ultrahigh enhancement factor of 1.86 × 106. Utilizing the Kelvin probe force microscopy (KPFM) technology, we found that these 2D a-TiO2 NSs possessed more positive surface potential than their 2D crystalline counterpart (c-TiO2 NSs). First-principles density functional theory (DFT) was used to further reveal that the low coordination number of surface Ti atoms and the large amount of surface oxygen defects endowed the 2D a-TiO2 with high electrostatic potential, which allowed significant charge transfer from the adsorbed molecule to the 2D a-TiO2 and facilitated the formation of a stable surface charge-transfer (CT) complex. Significantly, comparing with the 2D c-TiO2, the smaller band gap and higher electronic density of states (DOS) of the 2D a-TiO2 effectively enhanced the vibronic coupling of resonances in the substrate-molecule system. The strong vibronic coupling within the CT complex obviously enhanced the PICT resonance and lead to the remarkable SERS activity of a-TiO2 NSs. To the best of our knowledge, this is the first report on the remarkable SERS activity of 2D amorphous semiconductor nanomaterials, which may bring the cutting edge of development of stable and highly sensitive nonmetal SERS technology.

High-quality Temperature Sensor Based on the Plasmonic Resonant Absorber

Aiming at the achievement of temperature measurement with high sensitivity in the sub-wavelength scale, an all-metal meta-surface (AMMS)-based sensor is numerically demonstrated. Based on the sharp plasmonic resonances and the simultaneous use of perfect absorption response, temperature sensing with an ultra-high enhancement factor of 263 for the spectral figure of merit (FOM) factor is obtained in comparison with that of the common plasmonic sensors. Moreover, spectral shift-related sensitivity S is up to 0.274 nm/°C. These sensing properties indicate the designed sensor can play a significant role in optoelectronic devices for high-integrated and high signal-to-noise ratio temperature detection.

Ultrasensitive Surface-Enhanced Raman Spectroscopy Detection Based on Amorphous Molybdenum Oxide Quantum Dots.

Surface-enhanced Raman spectroscopy (SERS) based on plasmonic semiconductive material has been proved to be an efficient tool to detect trace of substances, while the relatively weak plasmon resonance compared with noble metal materials restricts its practical application. Herein, for the first time a facile method to fabricate amorphous Hx MoO3 quantum dots with tunable plasmon resonance is developed by a controlled oxidization route. The as-prepared amorphous Hx MoO3 quantum dots show tunable plasmon resonance in the region of visible and near-infrared light. Moreover, the tunability induced by SC CO2 is analyzed by a molecule kinetic theory combined with a molecular thermodynamic model. More importantly, the ultrahigh enhancement factor of amorphous Hx MoO3 quantum dots detecting on methyl blue can be up to 9.5 × 105 with expending the limit of detection to 10-9 m. Such a remarkable porperty can also be found in this Hx MoO3 -based sensor with Rh6G and RhB as probe molecules, suggesting that the amorphous Hx MoO3 quantum dot is an efficient candidate for SERS on molecule detection in high precision.

Remarkable SERS Activity Observed from Amorphous ZnO Nanocages.

Enhancement of the semiconductor-molecule interaction, in particular, promoting the interfacial charge transfer process (ICTP), is key to improving the sensitivity of semiconductor-based surface enhanced Raman scattering (SERS). Herein, by developing amorphous ZnO nanocages (a-ZnO NCs), we successfully obtained an ultrahigh enhancement factor of up to 6.62×105 . This remarkable SERS sensitivity can be attributed to high-efficiency ICTP within a-ZnO NC molecule system, which is caused by metastable electronic states of a-ZnO NCs. First-principles density functional theory (DFT) simulations further confirmed a stronger ICTP in a-ZnO NCs than in their crystalline counterparts. The efficient ICTP can even generate π bonding in Zn-S bonds peculiar to the mercapto molecule adsorbed a-ZnO NCs, which has been verified through the X-ray absorption near-edge structure (XANES) characterization. To the best of our knowledge, this is the first time such remarkable SERS activity has been observed within amorphous semiconductor nanomaterials, which could open a new frontier for developing highly sensitive and stable SERS technology.

Remarkable SERS Activity Observed from Amorphous ZnO Nanocages

AbstractEnhancement of the semiconductor–molecule interaction, in particular, promoting the interfacial charge transfer process (ICTP), is key to improving the sensitivity of semiconductor‐based surface enhanced Raman scattering (SERS). Herein, by developing amorphous ZnO nanocages (a‐ZnO NCs), we successfully obtained an ultrahigh enhancement factor of up to 6.62×105. This remarkable SERS sensitivity can be attributed to high‐efficiency ICTP within a‐ZnO NC molecule system, which is caused by metastable electronic states of a‐ZnO NCs. First‐principles density functional theory (DFT) simulations further confirmed a stronger ICTP in a‐ZnO NCs than in their crystalline counterparts. The efficient ICTP can even generate π bonding in Zn−S bonds peculiar to the mercapto molecule adsorbed a‐ZnO NCs, which has been verified through the X‐ray absorption near‐edge structure (XANES) characterization. To the best of our knowledge, this is the first time such remarkable SERS activity has been observed within amorphous semiconductor nanomaterials, which could open a new frontier for developing highly sensitive and stable SERS technology.

Flexible SERS active detection from novel Ag nano-necklaces as highly reproducible and ultrasensitive tips

Surface-enhanced Raman scattering (SERS) spectroscopy has been considered as a promising way to realize real-time, in-situ and ultrasensitive analysis of chemoand biochemical molecules in different applications even in intracellular or aqueous environments. In this work, polymer-supported novel Ag nano-necklaces (AgNLs) as flexible SERS substrates were fabricated for ultrasensitive chemical and biological detection. With the stringing of dense “hot spot” in three-dimension, AgNLs located on polydimethylsiloxane (PDMS) work like the removable and reusable “tip” on the surface of analytes with different morphologies and conditions. The novel substrate shows ultra-high enhancement factor (as high as 109) with excellent reproducibility and long-term stability (7 months) in an aqueous environment. With further functionalizing with p-mercaptobenzoic acid, AgNLs/PDMS elastomer also reveals sensitive and consistent pH detection ability over the wide range of pH 4.0–9.0, indicating their wide applications in biological and environmental fields. This work provides a feasible strategy for designing ultrasensitive, reproducible and flexible SERS substrate for practical detection.

Ultrahigh Enhancement Factor by Using a Silver Nanoshell With a Gain Core Above a Silver Substrate for Surface-Enhanced Raman Scattering at the Single-Molecule Level

Single-molecule surface-enhanced Raman scattering (SERS) has been extensively studied over the past decade. In this paper, we propose a novel method to obtain ultrahigh enhancement of local electric fields by using a silver nanoshell with a gain core above a silver substrate. Numerical results show that an ultrahigh enhancement factor of the local electric field on the order of 10 5 -10 6 and a SERS enhancement factor on the order of 10 20 -10 25 can be obtained with the present system. The SERS enhancement factor is five orders of magnitude greater than the highest theoretical value reported for SERS applications at the single-molecule level. In addition, we use the finite-element method to analyze in detail the influences of the gain coefficient of the gain core and the other structure parameters on the enhancement factor.

Fabricating Silver Nanoparticles on Thin Silicon Nanowalls for Highly Sensitive Surface-Enhanced Raman Scattering

Metal nanoparticles with nanoscale spacing are promising materials for the detection of single molecules through surface-enhanced Raman scattering. To increase the sensitivity of nanoparticles through the use of a nanoscale substrate, we fabricated various Ag NP­decorated silicon nanowalls for the Raman spectroscopic detection of rhodamine 6G (R6G). The sensitivity of detection was affected by the nanowall depth and was influenced by several parameters: the AgNO3 concentration for metal-assisted etching, the HF/H2O2 etching time for nanowall formation, and the Ag evaporation time for nanoparticle growth. For an approximately 400-nm-deep nanowall substrate having the optimal surface filling ratio and etching depth, we obtained an ultrahigh enhancement factor of 1.1 © 10 9 for the detection of R6G at a concentration of 10 ¹11 M.