Elucidation of charge-transfer SERS selection rules by considering the excited state properties and the role of electrode potential

Toward Modeling the Complexity of the Chemical Mechanism in SERS

Surface-enhanced Raman scattering (SERS) provides detailed information about the binding of molecules at interfaces and their interactions with the local environment due to the large enhancement of Raman scattering. This enhancement arises from a combination of the electromagnetic mechanism (EM) and chemical mechanism (CM). While it is commonly accepted that EM gives rise to most of the enhancement, large spectral changes originate from CM. To elucidate the rich information contained in SERS spectra about molecules at interfaces, a comprehensive understanding of the enhancement mechanisms is necessary. In this Perspective, we discuss the current understanding of the enhancement mechanisms and highlight their interplay in complex local environments. We will also discuss emerging areas where the development of computational and theoretical models is needed with specific attention given to how the CM contributes to the spectral changes. Future efforts in modeling should focus on overcoming the challenges presented in this review in order to capture the complexity of CM in SERS.

Linear Vibronic Coupling Approach for Surface-Enhanced Raman Scattering: Quantifying the Charge-Transfer Enhancement Mechanism

The outstanding amplification observed in surface-enhanced Raman scattering (SERS) is due to several enhancement mechanisms, and standing out among them are the plasmonic (PL) and charge-transfer (CT) mechanisms. The theoretical estimation of the enhancement factors of the CT mechanism is challenging because the excited-state coupling between bright plasmons and dark CT states must be properly introduced into the model to obtain reliable intensities. In this work, we aim at simulating electrochemical SERS spectra, considering models of pyridine on silver clusters subjected to an external electric field E⃗ that represents the effect of an electrode potential Vel. The method adopts quantum dynamical propagations of nuclear wavepackets on the coupled PL and CT states described with linear vibronic coupling models parametrized for each E⃗ through a fragment-based maximum-overlap diabatization. By presenting results at different values of E⃗, we show that indeed there is a relation between the population transferred to the CT states and the total scattered intensity. The tuning and detuning processes of the CT states with the bright PLs as a function of the electric field are in good agreement with those observed in experiments. Finally, our estimations for the CT enhancement factors predict values in the order of 105 to 106, meaning that when the CT and PL states are both in resonance with the excitation wavelength, the CT and PL enhancements are comparable, and vibrational bands whose intensity is amplified by different mechanisms can be observed together, in agreement with what was measured by typical experiments on silver electrodes.

Recent advances in optical aptasensors for biomarkers in early diagnosis and prognosis monitoring of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) accounts for approximately 90% of all primary liver cancers and is one of the main malignant tumor types globally. It is essential to develop rapid, ultrasensitive, and accurate strategies for the diagnosis and surveillance of HCC. In recent years, aptasensors have attracted particular attention owing to their high sensitivity, excellent selectivity, and low production costs. Optical analysis, as a potential analytical tool, offers the advantages of a wide range of targets, rapid response, and simple instrumentation. In this review, recent progress in several types of optical aptasensors for biomarkers in early diagnosis and prognosis monitoring of HCC is summarized. Furthermore, we evaluate the strengths and limitations of these sensors and discuss the challenges and future perspectives for their use in HCC diagnosis and surveillance.

Computational Model for Electrochemical Surface-Enhanced Raman Scattering: Key Role of the Surface Charges and Synergy between Electromagnetic and Charge-Transfer Enhancement Mechanisms

We present a computational model for electrochemical surface-enhanced Raman scattering (EC-SERS). The surface excess of charge induced by the electrode potential (V_el) was introduced by applying an external electric field to a set of clusters [Ag_n]^q with (n, q) of (19, ±1) or (20, 0) on which a molecule adsorbs. Using DFT/TD-DFT calculations, these metal-molecule complexes were classified by the adsorbate partial charge, and the main V_el-dependent properties were simultaneously studied with the aid of vibronic resonance Raman computations, namely, changes on the vibrational wavenumbers, relative intensities, and enhancement factors (EFs) for all SERS mechanisms: chemical or nonresonant, and resonance Raman with bright states of the adsorbate, charge-transfer (CT) states, and plasmon-like excitations on the metal cluster. We selected two molecules to test our model, pyridine, for which V_el has a remarkable effect, and 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene, which is almost insensitive to the applied bias. The results nicely reproduced most of the experimental observations, while the limitations of our approach were critically evaluated. We detected that accounting explicitly for the surface charges is key for EC-SERS models and that the highest calculated EFs, up to 10⁷ to 10⁸, are obtained by interstate coupling of bright local excitations of the metal cluster and CT states. These results highlight the importance of nonadiabatic effects in SERS and the capabilities of EC-SERS as a technique with potential to study excited-state coupling by tuning the CT and plasmon-like states by manipulating V_el.

Highly sensitive detection of cytochrome c in the NSCLC serum using a hydrophobic paper based-gold nanourchin substrate

Cytochrome c (Cyt c) is a biomarker of early apoptosis that plays a critical role in the diagnosis and therapy of non-small cell lung cancer (NSCLC). In this work, we proposed a novel surface-enhanced Raman scattering (SERS)-based biosensor to implement the ultrasensitive detection of Cyt c in the serum of NSCLC patients. The SERS-supporting substrates based on hydrophobic filter paper were composed of gold nanourchins (GNUs) surface-functionalized with the Cyt c aptamer and the cyanine 5-labeled complementary DNA. In the existence of Cyt c, it could specifically bind to its aptamer, which leads to the detachment of complementary strands modified with Cy5 and the great weakness of SERS signal. The finite-difference time domain (FDTD) simulation showed that the excellent SERS performance of GNUs aggregation was strongly dependent on a large number of "hot spots" at the tips and between the nanogaps of aggregated GNUs. Alkyl ketene dimer (AKD) was used to make the filter paper modify its property from hydrophilic to hydrophobic, which consequently increased the density of GNUs and extended the retention time of the analyte. SERS biosensors based on hydrophobic paper exhibited prominent reproducibility and selectivity. The detection limit of Cyt c in PBS was 1.148 pg/mL, while the detection limit in human serum was 1.79 pg/mL. Moreover, the analysis of the serum samples of healthy subjects and NSCLC patients confirmed the feasibility of its clinical application. The results were consistent with enzyme-linked immunosorbent assay results. This method can be a powerful strategy for quantitative detection of extracellular Cyt c, and it is expected that the SERS-based biosensors could be applied in the practical clinical diagnoses of NSCLC.

Relative contributions of Franck-Condon to Herzberg-Teller terms in charge transfer surface-enhanced Raman scattering spectroscopy

We have theoretically modeled charge transfer (CT) surface enhanced raman scattering (SERS) spectroscopy using pyridine bound to a planar Ag₆ metal nanocluster. CT states were determined by natural transition orbital hole-particle plots and CT distance D_CT and the amount of charge transferred q_CT indices. We first consider a resonance Raman (RR) model based on the Albrecht approach and calculate the ratio of the Herzberg-Teller (HT) B or C term to the Franck-Condon (FC) A term for a totally symmetric a₁ vibrational mode exciting in the lowest energy CT state. Using a dimensionless upper limit to the displacement factor ∆ = 0.05 in the FC term based on the examination of overtones in experimental spectra and a calculated HT coupling constant h_CT = 0.439 eV/Å(amu)^1/2 in the HT term, we calculated the scattering ratio of the HT to FC intensities as 147. This example indicated that for totally symmetric modes, the scattering intensity would all come from HT scattering. To further verify this result, we used the general time-dependent-RR formulation of Baiardi, Bloino, and Barone with the adiabatic Hessian model to calculate the FC, the Frank-Condon and Herzberg-Teller (FCHT), and the HT terms for pyridine in the C_2v Ag₆-pyridine complexes. For all cases we studied with pyridine in two orientations either parallel or perpendicular to the planar Ag₆ cluster, the HT terms, FCHT + HT, dominate the FC term in the CT RR spectrum. These results indicate that for CT SERS, the intensity of all the totally and non-totally symmetric vibrational modes should come from the HT effect.

Charge-transfer surface-enhanced resonance Raman spectra of benzene-like derivative compounds under the effect of an external electric field

Since the discovery of surface-enhanced resonance Raman scattering (SERS), elucidating the charge-transfer (CT) mechanism has been a challenging and controversial process. Different theoretical models have been proposed to explain the effect of applied electrode potential on SERS-CT, but achieving a high-quality conserved trend of experimental observations and explaining the nature of the selective enhancement of the signals is not a trivial task and the results and conclusions are still in dispute. We investigated recently the performance of time-dependent excited-state gradient approximation under the effects of a uniform finite electric field in a simulation of the experimental spectra of pyridine on an Ag electrode. The singular patterns of the experimental spectra for symmetric and non-symmetric benzene-like derivative compounds and the consistent trends of enhancements of their signals under various electrode potentials motivated us to extend our simulation studies to 4-methylpyridine, pyrazine and pyrimidine molecules on silver metal clusters. For these molecules, selective enhancement and de-enhancement of totally symmetric (υ_6a, υ_9a and υ_8a) and non-totally symmetric (υ_6b and υ_8b) modes upon changing the field were obtained and matched well with experimental observations. The selective enhancement of each signal in a zero field was explained by means of excited-state vector gradients and excited-state charge density difference for the S₀→ S_CT transition. On-field calculations showed slight perturbations of the geometries and electronic structures of the molecules. These on-field calculations also directly affected the magnitude of specific excited-state vector gradients and dimensionless displacements, and moreover the patterns of the spectra. The results of this investigation provided insight into the nature of the selective enhancements of signals and may help researchers propose the selection rules of SERS-CT.

Theoretical Approaches for Modeling the Effect of the Electrode Potential in the SERS Vibrational Wavenumbers of Pyridine Adsorbed on a Charged Silver Surface

Vibrational wavenumbers of pyridine adsorbed on a silver electrode have been correlated to the calculated ones from different theoretical approaches based on DFT methods. The vibrational tuning caused by the electrode potential has been simulated by means of pyridine-silver clusters with different densities of charge or, alternatively, under applied external electric fields. Both methodologies predict correctly a qualitative red-shift of the vibrational wavenumbers at negative potentials. As a result, harmonic frequency calculations performed at the B3LYP/LanL2DZ level of theory by using a linear [Ag_nPy]^q complex model with different densities of charge (q_eff = q/n) have exhibited the best agreement with the experimental observations although the tuning amplitudes are overestimated. Electric fields calculations are unable to account for subtle details observed in the spectra related to the differentiated chemical nature of the metal-molecule bond at positive or negative potentials with respect to the potential of zero charge of the electrode.

SERS investigation and high sensitive detection of carbenicillin disodium drug on the Ag substrate

The reliable and ultrasensitive detection of antibiotic drug residue is of great interest for environmental protection and human health. Herein, we propose a simple SERS strategy based on Ag nanoparticles (NPs) as substrate with the assistance of aggregation agent (MgSO₄) for the SERS investigation and the high sensitive detection of antibiotic drug carbenicillin disodium (CBDM). The density functional theory calculation was performed for the assignment and identification of Raman bands of the CBDM molecule. The results indicate that the CBDM molecule is close to the Ag NP substrate surface through the carboxyl group. The CBDM molecules on Ag NP substrate exhibit the largest SERS enhancement, when the concentration of MgSO₄ is 1 × 10^-2 mol/L and the pH value of CBDM solution is 6. By this SERS method, the limit of detection of CBDM is 0.63 × 10^-8 mol/L, which is lower than the standard of European Union for the maximum residue limit of antibiotic drug (1.2 × 10^-8 mol/L). And, a quantitative detection method of CBDM can be established. There is a good linear relationship (R² = 0.9908) in the concentration range of 1.0 × 10^-8-1.0 × 10^-3 mol/L. It proves that the proposed SERS method is a simple, rapid (within 6 min), reliable and highly sensitive scheme with a good reproducibility for the detection of CBDM. And, the proposed SERS strategy can also be applied for the high sensitive detection and identification of other antibiotic drug (penicillin).

Reply to the 'Comment on "Elucidation of charge-transfer SERS selection rules by considering the excited state properties and the role of electrode potential"' by M. Mohammadpour, M. H. Khodabandeh, L. Visscher and Z. Jamshidi, Phys. Chem. Chem. Phys., 2017, 19, 7833

We appreciate Aranda's comments on our recent work entitled ''Elucidation of charge-transfer SERS selection rules by considering the excited state properties and the role of electrode potential''. We would also like to thank the editor of Physical Chemistry Chemical Physics for giving us an opportunity to specify more details of our work in this reply. An important part of our article concerns the role of the electrode potential in charge-transfer SERS spectra and we would like to first address the questions that Aranda et al. posed about our labeling.

Comment on “Elucidation of charge-transfer SERS selection rules by considering the excited state properties and the role of electrode potential” by M. Mohammadpour, M. H. Khodabandeh, L. Visscher and Z. Jamshidi, Phys. Chem. Chem. Phys., 2017, 19, 7833

Different theoretical tools for modelling the complex role of the electrode potential in SERS are highlighted.

Theoretical modeling of voltage effects and the chemical mechanism in surface-enhanced Raman scattering

Theoretical approaches can provide insight into the mechanisms and magnitudes of electromagnetic and chemical effects in surface-enhanced Raman scattering (SERS), properties that are not readily available experimentally. Here, we model the SERS spectra of two geometries of the prototypical Ag₂₀–pyridine cluster using a semiempirical INDO/SCI approach that allows a straightforward decomposition of the enhancement factors at each wavelength into electromagnetic and chemical terms, with proper treatment of resonant charge-transfer contributions to the enhancement. The method also enables us to determine the dependence of the enhancement on the electrochemical potential. We show that the electromagnetic enhancements for the Ag₂₀ cluster are <10 far from resonance but can increase to 10² to 10³ on resonance with plasmon excitation in the cluster. The decomposition also shows that for the systems studied here, the chemical enhancements are primarily due to resonance with excited states with significant charge-transfer character. This term is typically <10 but can be >10² at electrochemical potentials where the charge-transfer excited states are resonant with the incoming light, leading to total enhancements of >10⁴.