SERS‐based assays for sensitive detection of modafinil

In this paper, we proposed two sensitive surface‐enhanced Raman spectroscopy assays for the determination of modafinil in urine matrix. In the first assay, modafinil was extracted by solid phase extraction and determined after sandwiching with silver nanoparticles. In the second assay, modafinil was extracted by non‐specific interaction with magnetic gold nanoparticles and determined by sandwiching with magnetic gold and silver nanoparticles. The non‐specific magnetic extraction does not require any analyte specific agent like antibody, aptamer, or molecularly imprinted polymer, therefore, the cost and complexity of the assay is very low. Both assays are capable for application to urine samples with the detection limits under the minimum required performance limit of modafinil. The assays were validated in terms of accuracy, precision, detection limits, and ranges. Copyright © 2015 John Wiley & Sons, Ltd.     

High performance Au/Ag core/shell bipyramids for determination of thiram based on surface‐enhanced Raman scattering

Au/Ag core/shell bipyramids were used as surface‐enhanced Raman scattering (SERS) substrates to determine the thiram. The metallic substrates showed high SERS performance and are very suitable for the analytical sensors. The fabrication and characterization of the Au/Ag core/shell bipyramids were described. The influence of experimental parameters, such as the thickness of Ag shell of the bipyramids, sodium chloride concentration, and pH value on SERS of thiram was examined and optimized. Under the optimum conditions, thiram molecules were effectively adsorbed onto bipyramids and the SERS intensity is proportional to the concentration of thiram in the range of 3.3 to 400.0 ng mL^–1. The corresponding correlation coefficient of the linear equation is 0.997, which indicates that there is a good linear relationship between SERS intensity and thiram concentration. The limit of detection for thiram is 2.0 ng mL^–1. The experimental results indicate that the proposed method is a viable method for determination of thiram. Some environmental water samples were analyzed and the analytical results were satisfactory. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis of ultrathin and compact Au@MnO2 nanoparticles for shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS)

Shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS) based on Au@SiO₂ or Au@Al₂O₃ nanoparticles (NPs) shows great potential to break the long‐standing limitations of substrate and surface generality of surface‐enhanced Raman scattering (SERS). However, the shell of SiO₂ or Al₂O₃ can easily be dissolved in alkaline media, which limits the applications of SHINERS in alkaline systems. Besides that, the synthesis of Au@SiO₂ NPs can be further simplified and Au@Al₂O₃ NPs be replaced by other NPs that are more amenable for mass production. In an attempt to make SHINERS NPs available in any systems practically, we report the synthesis of ultrathin and compact Au@MnO₂ NPs. The shell thickness of MnO₂ can be controlled down to about 1.2 nm without any pinhole. SHINERS based on such Au@MnO₂ NPs exhibits much higher Raman enhancement effect than Au@SiO₂ NPs and can be applied in alkaline systems in which Au@SiO₂ or Au@Al₂O₃ NPs cannot be applied. Copyright © 2011 John Wiley & Sons, Ltd.     

Rapid synthesis and characterization of ultra‐thin shell Au@SiO2 nanorods with tunable SPR for shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS)

The recently reported shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS) is considered as the next generation of advanced spectroscopy for its surface and molecular generality. With the aim to utilize the virtues of shell‐isolated strategy and advance the SHINERS technique, we introduce a silane‐based rapid synthesis method of silica‐coating Au nanorods (Au@SiO₂ NRs) with manoeuvrable ultra‐thin shell and tunable SPR. The results demonstrate that the SPR of Au NRs could be optimized to obtain large Raman enhancement using either 633 nm or 785 nm laser. Differing from previously reported Au@SiO₂ NRs synthesis method, we can tune the silica shell thickness within several nanometers to maximize the Raman signal while effectively eliminating the exterior interference. And this advanced synthesis method has also significantly reduced the silica‐coating time from one day to ca. 1 h. This method as a new development of SHINERS technique has successfully got enhanced signal in solution Raman tests of malachite green, giving a great potential to be extended to in‐situ measurement for daily life detection. Copyright © 2013 John Wiley & Sons, Ltd.     

Ultra sensitive surface‐enhanced Raman scattering detection based on monolithic column as a new type substrate

A novel ultrasensitive detection method utilizing surface‐enhanced Raman scattering (SERS) based on monolithic column was developed in the present study. Monolithic column is a kind of chromatographic stationary phase that contains highly interconnected pores and absorbs chemical components efficiently. Dropping a mixture solution containing analyte, silver colloid, and NaCl on the surface in advance, SERS signals were collected on the surface of the monolithic column. With this method, five commonly used probe molecules of Rhodamine 6G (R6G), p‐aminothiophenol, Rhodamine 123, crystal violet, thymine, and two chemicals that are used in agriculture (paraquat and flusilazole) were detected. Especially, R6G and p‐aminothiophenol can be detected at extremely low concentrations of 10^–18 and 10^–16 mol/L at milliliter level, respectively. The enhancement factor was calculated to be approximately 10¹⁴ for R6G detection. The results suggest that the monolithic column does improve the sensitivity of SERS detection dramatically and the topography of the monolithic column is essential for the enhancement. The easy operability and the significant enhancement are the greatest advantages of this method. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and Raman signature for the formation of CdO/MnO2 (core/shell) nanostructures

In the present report, bare CdO and CdO/MnO₂ core/shell nanostructures of various cores and different shell sizes were synthesized using co‐precipitation method. The phase, size, shape and structural details of the bare CdO and CdO/MnO₂ nanostructures were investigated by X‐ray diffraction, transmission electron microscopy (TEM), and Raman spectroscopy measurements. TEM micrographs confirm the formation of core/shell nanostructures. The presence of CdO (core) and MnO₂ (shell) crystal phases was determined by analyzing the Raman data of bare CdO and CdO/MnO₂ core/shell nanostructures. The Raman spectra of bare CdO nanostructures contain one broad intense convoluted envelop of three bands in the spectral range of 200–500 cm⁻¹ and a weaker band located at ~940 cm⁻¹. The intensity of these two Raman bands is decreased with the increase of shell size and disappeared completely for the shell size 5.3 ± 1 nm. Further, two new Raman bands appeared at ~451 and ~665 cm⁻¹ for the shell size 1.3 ± 0.1 nm. These two Raman bands are assigned to the deformation of Mn–O–Mn and Mn–O stretching modes of MnO₂. The intensity of these two Raman bands is enhanced with the increase of shell size and attains a maximum value for the shell size 5.3 ± 1 nm. The disappearance of characteristics Raman bands of CdO phase and the appearance of characteristics Raman bands corresponding to MnO₂ phase for nanostructures of shell size 5.3 ± 1 nm authenticate the presence of CdO as core and MnO₂ as shell in the core/shell nanostructures. Copyright © 2014 John Wiley & Sons, Ltd.     

Development of a sensitive,stable and EGFR‐specific molecular imaging agent for surface enhanced Raman spectroscopy

We created and studied a novel nanoprobe for spectroscopic molecular imaging of the epidermal growth factor (EGF) receptor, whose over‐expression is a hallmark of a wide range of cancers. Silver nanoparticles (AgNPs) of 45 nm diameter were synthesized and coupled to EGF by α‐lipoic acid, a short ligand that exhibits excellent silver binding affinity. Time‐of‐flight mass spectroscopy demonstrates formation of the protein complex. Enzyme‐linked immunosorbent assay verifies the protein complex is 100% active for the EGF receptor, alone and, following conjugation to silver nanoparticles. Compared with its monosulfide analog, 6‐mercaptohexanoic acid, α‐lipoic acid is stabilized by binding to silver with a total energy that is lower by 1.38 eV, as found from Density Functional Theory (DFT)/natural bond analysis calculations. A Highest Occupied Molecular Orbital (HOMO)‐Lowest Unoccupied Molecular Orbital (LUMO) gap energy of 5.25 (spin‐up electrons) and 5.74 eV (spin‐down electrons) was obtained for the silver‐α‐lipoic acid complex. This is the first report of silver nanoparticles being attached to EGF, and the first theoretical and experimental report on the surface enhanced Raman spectroscopy spectral interpretation of α‐lipoic acid bound to silver. These nanoprobes exhibit surface enhanced Raman spectroscopy, when aggregated in solution, at picomolar concentrations and have the necessary properties – specificity, sensitivity and stability – to serve as molecular imaging agents. Copyright © 2015 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy for quantitative measurement of lactic acid at physiological concentration in human serum

Lactic acid is a simple and effective indicator for estimating physiological function. Rapid and sensitive detection of lactic acid is very useful in clinical diagnosis. However, the concentration of lactic acid in the physiological state is too low to be detected using traditional Raman spectroscopy. We applied silver colloidal nanoparticles‐mediated surface‐enhanced Raman spectroscopy (SERS) for rapid identification and quantification of lactic acid. The standard SERS spectra of lactic acid were defined and the 1395 cm⁻¹ band intensity was used for quantification from 0.3 to 2 mM (R² = 0.99). In clinical blood sample measurement, the ultrafiltration (cutoff value 5 kDa) can efficiently reduce background fluorescence to improve SERS performance. We established identical and optimal procedure by adjusting reaction time and volume ratio of serum and nanoparticles to obtain high SERS reproducibility. Finally, we showed that silver colloidal nanoparticles‐mediated SERS technique was successfully applied to detect lactic acid at physiological concentrations in the blood. Copyright © 2010 John Wiley & Sons, Ltd.     

Joint theoretical and experimental study of the gas‐phase elimination kinetics of tert‐butyl ester of carbamic,N, N‐dimethylcarbamic,N‐hydroxycarbamic acids and 1‐(tert‐butoxycarbonyl)‐imidazole

The gas‐phase elimination kinetics of the title compounds were carried out in a static reaction system and seasoned with allyl bromide. The working temperature and pressure ranges were 200–280 °C and 22–201.5 Torr, respectively. The reactions are homogeneous, unimolecular, and follow a first‐order rate law. These substrates produce isobutene and corresponding carbamic acid in the rate‐determining step. The unstable carbamic acid intermediate rapidly decarboxylates through a four‐membered cyclic transition state (TS) to give the corresponding organic nitrogen compound. The temperature dependence of the rate coefficients is expressed by the following Arrhenius equations: for tert‐butyl carbamate logk₁ (s^?1) = (13.02 ± 0.46) – (161.6 ± 4.7) kJ/mol(2.303 RT)^?1, for tert‐butyl N‐hydroxycarbamate logk₁ (s^?1) = (12.52 ± 0.11) – (147.8 ± 1.1) kJ/mol(2.303 RT)^?1, and for 1‐(tert‐butoxycarbonyl)‐imidazole logk₁ (s^?1) = (11.63 ± 0.21)–(134.9 ± 2.0) kJ/mol(2.303 RT)^?1. Theoretical studies of these elimination were performed at Møller–Plesset MP2/6‐31G and DFT B3LYP/6‐31G(d), B3LYP/6‐31G(d,p) levels of theory. The calculated bond orders, NBO charges, and synchronicity (Sy) indicate that these reactions are concerted, slightly asynchronous, and proceed through a six‐membered cyclic TS type. Results for estimated kinetic and thermodynamic parameters are discussed in terms of the proposed reaction mechanism and TS structure. Copyright © 2007 John Wiley & Sons, Ltd.     

Identification of two Ag‐2,2′:6′,2″‐terpyridine surface species on Ag nanoparticle surfaces by excitation wavelength dependence of SERS spectra and factor analysis: evidence for chemical mechanism contribution to SERS of Ag(0)–tpy

Measurement and interpretation of the excitation wavelength dependence of surface‐enhanced Raman scattering (SERS) spectra of molecules chemisorbed on plasmonic, e.g. Ag nanoparticle (NP) surfaces, are of principal importance for revealing the charge transfer (CT) mechanism contribution to the overall SERS enhancement. SERS spectra, their excitation wavelength dependence in the 445–780‐nm range and factor analysis (FA) were used for the identification of two Ag‐2,2′:6′,2″‐terpyridine (tpy) surface species, denoted Ag⁺–tpy and Ag(0)–tpy, on Ag NPs in systems with unmodified and/or purposefully modified Ag NPs originating from hydroxylamine hydrochloride‐reduced hydrosols. Ag⁺–tpy is a spectral analogue of [Ag(tpy)]⁺ complex cation, and its SERS shows virtually no excitation wavelength dependence. By contrast, SERS of Ag(0)–tpy surface complex generated upon chloride‐induced compact aggregate formation and/or in strongly reducing ambient shows a pronounced excitation wavelength dependence attributed to a CT resonance (the chemical mechanism) contribution to the overall SERS enhancement. Both the resonance (λ_exc = 532 nm) and off‐resonance (λ_exc = 780 nm) pure‐component spectra of Ag(0)–tpy obtained by FA are largely similar to surface‐enhanced resonance Raman scattering (λ_exc = 532 nm in resonance with singlet metal to ligand CT (¹ MLCT) transition) and SERS (λ_exc = 780 nm) spectra of [Fe(tpy)₂]²⁺ complex dication. Copyright © 2014 John Wiley & Sons, Ltd.     

Double proton transfer in crystals of 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a] pyrimidine (hppH): 13C and 15N CPMAS NMR study of (hppH)2

In this paper, we report an example of intermolecular solid‐state proton transfer in the bicyclic guanidine, hppH. A combination of X‐ray crystallography, CPMAS NMR (¹³C and ¹⁵N) and theoretical calculations allows us to determine that a double proton transfer takes place in the (hppH)₂ dimer with an activation energy of about 50 kJ mol^?1. According to the B3LYP/6‐311++G(d,p) calculations, the double proton transfer occurs non‐symmetrically through a zwitterion. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental vibrational spectra (Raman,infrared) and DFT calculations on monomeric and dimeric structures of 2‐ and 6‐bromonicotinic acid

In this work, the Fourier transform infrared and Raman spectra of 2‐bromonicotinic acid and 6‐bromonicotinic acid (abbreviated as 2‐BrNA and 6‐BrNA, C₆H₄BrNO₂) have been recorded in the region 4000–400 and 3500–50 cm⁻¹. The optimum molecular geometry, normal mode wavenumbers, infrared intensities and Raman scattering activities, corresponding vibrational assignments and intermolecular hydrogen bonds were investigated with the help of B3LYP density functional theory (DFT) method using 6‐311++G(d,p) basis set. Reliable vibrational assignments were made on the basis of total energy distribution (TED) calculated with scaled quantum mechanical (SQM) method. From the calculations, the molecules are predicted to exist predominantly as the C1 conformer. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering investigations on silver nanoparticles deposited on alumina and titania nanotubes: influence of the substrate material on surface‐enhanced Raman scattering activity of Ag nanoparticles

Silver nanoparticles deposited on various ‘inert’ porous materials (mainly Al₂O₃ and TiO₂) are often used as substrates for surface‐enhanced Raman scattering (SERS) measurements. In this study, we used the sputter deposition technique to cover tubular arrays of Al₂O₃ and TiO₂ with Ag nanoparticles. Raman spectra of pyridine (as a probe molecule) and of two selected dyes (5‐(4‐dimethylaminobenzylidene)rhodanine and 5‐(4‐(dimethylamino)benzylidene)‐3‐(3‐methoxypropyl)rhodanine) adsorbed on fabricated Ag/TiO₂‐n/Ti and Ag/Al₂O₃‐n/Al substrates were measured. We found that the SERS spectra of pyridine adsorbed on Ag nanoparticles deposited on an Al₂O₃‐n/Al substrate are distinctly different from those measured for an Ag/TiO₂‐n/Ti composite. Similar effects were observed for dyes adsorbed on the surface of both composites. The spectral differences between two kinds of composites (Ag/TiO₂‐n/Ti and Ag/Al₂O₃‐n/Al) are discussed in terms of (1) the modified electronic structure of the Ag nanoparticles due to their interaction with different substrate materials and (2) the different atomic topology of the metal particles thus deposited on the surfaces of the substrates. Composite samples were also studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to reveal their characteristic morphological and chemical features. Copyright © 2012 John Wiley & Sons, Ltd.     

Solvatochromism of 2‐(N,N‐dimethylamino)‐7‐nitrofluorene and the natural dye β‐carotene: application for the determination of solvent dipolarity and polarizability

The effects of solvents on chemical phenomena (rate and equilibrium constants, spectroscopic transitions, etc.) are conveniently described by solvation free‐energy relationships that take into account solvent acidity, basicity and dipolarity/polarizability. The latter can be separated into its components by manipulating the UV–vis spectra of two solvatochromic probes, 2‐(N,N‐dimethylamino)‐7‐nitrofluorene (DMANF) and a di‐(tert‐butyl)‐tetramethyl docosanonaen probe (ttbP9) whose synthesis is laborious and expensive. Recently, we have shown that the natural dye β‐carotene can be conveniently employed instead of ttbP9 for the determination of solvent polarizability (SP) of 76 molecular solvents and four ionic liquids. In the present work, we report the polarizabilities of further 24 solvents. Based on the solvatochromism of β‐carotene and DMANF, we have calculated solvent dipolarity (SD) for 103 protic and aprotic molecular solvents, and ionic liquids. The dependence of SD and SP on the number of carbon atoms in the acyl‐ or alkyl group of several homologous series (alcohols; 2‐alkoxyethanols; carboxylic acid‐ anhydrides, and esters, ionic liquids) is calculated and briefly discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

Calculated polar substituent effects on the stability of 4‐substituted(X) ‐ cub‐1‐yl and ‐bicyclo[2.2.2]oct‐1‐yl cations: a DFT study

The effects of substituents on the stability of 4‐substituted(X) cub‐1‐yl cations ( 2 ), as well as the benchmark 4‐substituted(X) bicyclo[2.2.2]oct‐1‐yl cation systems ( 7 ), for a set of substituents (X = H, NO₂, CN, NC, CF₃, COOH , F, Cl, HO, NH₂, CH₃, SiH₃, Si(CH₃)₃, Li, O^?, and NH) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6‐311 + G(2d,p) level of theory. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 2 / 7 and polar field/group electronegativity substituent constants (σ_F and σ_χ, respectively). The analysis reveals that the ΔE values of both systems are best described by a combination of both substituent constants. This highlights the distinction between through‐space and through‐bond electronic influences characterized by σ_F and σ_χ, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Calculated polar substituent effects on the stability of 3‐substituted(X) bicyclo[1.1.1]pent‐1‐yl cations and 4‐substituted(X) bicyclo[2.2.1]hept‐1‐yl cations: a DFT study

The effects of substituents on the stability of 3‐substituted(X) bicyclo[1.1.1]pent‐1‐yl cations (3) and 4‐substituted(X) bicyclo[2.2.1]hept‐1‐yl cations (4), for a set of substituents (X = H, NO₂, CN, NC, CF₃, CHO, COOH , F, Cl, HO, NH₂, CH₃, SiH₃, Si(CH₃)₃, Li, O^?, and NH₃⁺) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6‐311 + G(2d,p) and B3LYP/6‐31 + G (d) levels of theory, respectively. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 3/4 and polar field/group electronegativity substituent constants (σ_F and σ_χ, respectively). The analysis reveals that the ΔE values for both systems are best described by a combination of both substituent constants. The result highlights the importance of the σ_χ dependency of charge delocalization in these systems. Copyright © 2012 John Wiley & Sons, Ltd.     

A DFT‐GIAO and DFT‐NBO study of polar substituent effects on NMR 17O chemical shifts in some rigid polycyclic alkanes

¹⁷O NMR shieldings of 3‐substituted(X)bicyclo[1.1.1]pentan‐1‐ols ( 1 , Y = OH), 4‐substituted(X)bicyclo[2.2.2]octan‐1‐ols ( 2 , Y = OH), 4‐substituted(X)‐bicyclo[2.2.1]heptan‐1‐ols ( 3 , Y = OH), 4‐substituted(X)‐cuban‐1‐ols ( 4 , Y = OH) and exo‐ and endo‐ 6‐substituted(X)exo‐bicyclo[2.2.1]heptan‐2‐ols ( 5 and 6 , Y = OH, respectively), as well as their conjugate bases ( 1 – 6 , Y = O^?), for a set of substituents (X = H, NO₂, CN, NC, CF₃, COOH, F, Cl, OH, NH₂, CH₃, SiMe₃, Li, O^?, and NH) covering a wide range of electronic substituent effects were calculated using the DFT‐GIAO theoretical model at the B3LYP/6‐311 + G(2d, p) level of theory. By means of natural bond orbital (NBO) analysis various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated ¹⁷O SCS and polar field and group electronegativity substituent constants (σ_F and σ_χ, respectively) and also the NBO derived molecular parameters (oxygen natural charge, Q_n, occupation numbers of the oxygen lone pairs, n_o, and occupancy of the C? O antibonding orbital, σ*_CO(occup)). In the case of the alcohols ( 1 – 6 , Y = OH) the ¹⁷O SCS appear to be governed predominantly by the σ_χ effect of the substituent. Furthermore, the key determining NBO parameters appear to be n_o and σ*_CO(occup). Unlike the alcohols, the calculated ¹⁷O SCS of the conjugate bases ( 1 – 6 , Y = O^?), except for system 1 , do not respond systematically to the electronic effects of the substituents. An analysis of the SCS of 1 (Y = O^?) raises a significant conundrum with respect to their origin. Copyright © 2010 John Wiley & Sons, Ltd.     

Studies of the spin-Hamiltonian parameters, d–d transitions and defect structures for two tetragonal Cu centers in Ba2ZnF6:Cu crystal

Two theoretical methods, the perturbation theory method (PTM) and the complete diagonalization (of energy matrix) method (CDM), are applied to calculate the spin-Hamiltonian parameters (g-factors g, g and hyperfine structure constants A, A, obtained from electron paramagnetic resonance (EPR) spectra) and d–d transitions (obtained from optical spectra) for two tetragonal Cu²⁺ centers in Ba₂ZnF₆:Cu²⁺ crystals. The Cu²⁺(I) ion replaces the Zn²⁺ ion at tetragonally compressed octahedral coordination and has the ground state ²A₁(|d_z²), whereas the Cu²⁺(II) ion is at an interstitial site with a square-planar F⁻coordination and has the ground state ²B₂(|d_x²-y²). The calculated spin-Hamiltonian parameters and d–d transitions from the PTM and CDM coincide and are in reasonable agreement with the experimental values. This suggests that both methods are effective for the theoretical studies of EPR and optical spectral data for 3d⁹ ions in tetragonal symmetry with different ground states. The defect structures of the two Cu²⁺ centers in Ba₂ZnF₆:Cu²⁺ are also estimated.