Gold nanoparticle-based pH sensor in highly alkaline region at pH > 11: surface-enhanced Raman scattering study

A surface plasmon resonance spectroscopy study showed that citrate-reduced gold nanoparticles ( approximately 15 nm diameter, approximately 9 x 10(-9) M concentration, approximately 2 x 10(-2) M ionic strength) were found to be utilized as a colorimetric sensor by exhibiting a distinct color change at a highly alkaline pH > 11.5. Surface-enhanced Raman scattering (SERS) of 4-ethynylpyridine (4-EP) on gold nanoparticle surfaces indicated that the multiple peaks in the v(C identical withC) stretching bands should vary significantly in the highly alkaline region from pH 12 to 14. As the pH value increased, the v(C identical withC) stretching band intensity at approximately 2080 cm(-1) became stronger than that at approximately 2010 cm(-1). The pK(1/2) value was determined to be around 13 by the SERS titration of taking intensity ratios of I(2080) with respect to I(2010). Using SERS enhancements and conspicuous spectral changes, self-assembled monolayers (SAMs) of 4-EP on Au nanoparticles holds potential as a pH sensor for sensitive detection of the hydroxide OH(-) concentration at around pH 13 in an aqueous solution. The pH calibration from SERS titration of 4-EP is expected to have advantages in terms of higher alkaline detection limit and more precise measurements, if compared with the indigo carmine, the pK(1/2) value of which is 12.2.     

Surface-enhanced Raman spectra of calf thymus DNA adsorbed on concentrated silver colloid

Raman and surface-enhanced Raman scattering (SERS) spectra of calf thymus DNA were investigated. We have carried out improvements to the silver colloid preparation method of Lee and Meisel in two respects. In one method, the silver sol was boiled with rapid stirring for over two hours. In the second method, the silver sol was concentrated by centrifugation before adding it to the DNA solution. The resulting hydrosol could be stored for 15 months because of its high stabilization. Structural information with respect to the phosphate backbone, deoxyribose, and four bases of DNA could be obtained before and after the DNA solutions were added to the concentrated Ag colloid substrate. The intensities of almost all characteristic bands assigned to various groups of the components of DNA were enhanced to a remarkable degree. The enhancement effect of the DNA solution at neutral pH 7.0 was obviously much better than that at acidic pH 3.4 or at alkaline pH 8.5. Intensity increases of the SERS bands of the DNA solution with time were observed. The SERS signals obtained 16 hours after the interaction of the Ag colloid with the DNA solution were much better than the SERS signals obtained just after the mixed liquid was prepared. This method can be widely used to store the Ag colloid for long times and to obtain the SERS spectra of DNA molecules, and it can further be used to study the adsorption behavior of solute biomacromolecules in different solvents.     

Highly sensitive and selective determination of iodide and thiocyanate concentrations using surface-enhanced Raman scattering of starch-reduced gold nanoparticles

In this report, we propose a novel technique for the determination of the concentrations of iodide and thiocyanate by surface-enhanced Raman scattering (SERS) of starch-reduced gold nanoparticles. Starch-reduced gold nanoparticles show an intrinsic Raman peak at 2125 cm(-1) due to the -C≡C- stretching mode of a synthesized byproduct. Because of the high adsorptivity of iodide on a gold surface, the intensity of the SERS peak at 2125 cm(-1) decreases with an increase in the iodide concentration. Thiocyanate also strongly adsorbs on a gold surface, and a new peak appears at around 2100 cm(-1), attributed to the -C≡N stretching vibration in a SERS spectrum of starch-reduced gold nanoparticles. These two peaks were successfully used to determine the iodide and thiocyanate concentrations separately, even in their mixture system. The detection limit of this technique for iodide is 0.01 μM with a measurement range of 0.01-2.0 μM, while the detection limit of this technique for thiocyanate is 0.05 μM with a measurement range of 0.05-50 μM. This technique is highly selective for iodide and thiocyanate ions without interference from other coexisting anions such as other halides, carbonate, and sulfate.     

Separation free DNA detection using surface enhanced Raman scattering

Surface enhanced raman scattering (SERS) based molecular diagnostic assays for the detection of specific DNA sequences have been developed in recent years to compete with the more common fluorescence based approaches. Current SERS assays either require time-consuming separation steps that increase assay cost and can also increase the risk of contamination or they are negative assays, where the signal intensity decreases in the presence of target DNA. Herein, we report a new separation free SERS assay with an increase in signal intensity when target DNA is present using a specifically designed SERS primer. The presence of specific bacterial DNA from Staphylococcus epidermidis was detected using polymerase chain reaction (PCR) and SERS and indicates a new opportunity for exploration of SERS assays requiring minimal handling steps.     

Square wave voltammetric detection of chemical DNA damage with catalytic poly(4-vinylpyridine)-Ru(bpy)2(2+) films

Reversible, catalytic films of poly(4-vinylpyridine)-Ru(bpy)2(2+) [PVP-Ru(bpy)2(2+), bpy = 2,2'-bipyridine] on pyrolytic graphite (PG) electrodes were evaluated for the detection of damage to double-stranded (ds) DNA by using square wave voltammetry (SWV). Damage of both calf thymus and salmon testes ds-DNA in solution was induced by incubation of DNA at 37 degrees C with styrene oxide, the liver metabolite of styrene, and a suspected carcinogen. Both types of ds-DNA incubated in solution with saturated styrene oxide gave a linear increase in catalytic peak current up to 30 min, and an estimate of two damaged DNA bases in one thousand could be detected. The increase in catalytic current is attributed to better access of the catalyst redox sites to oxidizable bases in the damaged, partly unwound DNA. A self-contained "toxicity sensor" was also evaluated, which consisted of films of [PVP-Ru(bpy)2(2+)] on PG electrodes coated with films of ds-DNA and polydiallyldimethylammonium polycations assembled layer-by-layer. These films also gave an increase in catalytic peak current upon incubation in saturated styrene oxide, and an estimate of 1 damaged base in 1000 could be detected. Control films or solutions of ds-DNA treated in buffer or buffer containing unreactive toluene resulted in no significant changes in the catalytic peak current with incubation time.     

Selective photoelectrochemical detection of DNA with high-affinity metallointercalator and tin oxide nanoparticle electrode

Selective detection of double-stranded DNA (ds-DNA) in solution was achieved by photoelectrochemistry using a high-affinity DNA intercalator, Ru(bpy)2dppz (bpy = 2,2'-bipyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine) as the signal indicator and tin oxide nanoparticle as electrode material. When Ru(bpy)2dppz alone was irradiated with 470-nm light, anodic photocurrent was detected on the semiconductor electrode due to electron injection from its excited state into the conduction band of the electrode. The current was sustained in the presence of oxalate in solution, which acted as a sacrificial electron donor to regenerate the ground-state metal complex. After addition of double-stranded calf thymus DNA into the solution, photocurrent dropped substantially. The drop was attributed to the intercalation of Ru(bpy)2dppz into DNA and, consequently, the reduced mass diffusion of the indicator to the electrode, as well as electrostatic repulsion between oxalate anion and negative charges on DNA. The degree of signal reduction was a function of the DNA concentration, thus forming the basis for real-time DNA detection. The signal reduction was selective for ds-DNA, as no such effect was observed for single-stranded polynucleotides such as poly-G, poly-C, poly-A, and poly-U. The detection limit of calf thymus ds-DNA reached 1.8 x 10(-10) M in solution.     

Part II: surface-enhanced Raman spectroscopy investigation of methionine containing heterodipeptides adsorbed on colloidal silver

Surface-enhanced Raman scattering (SERS) spectra of methionine (Met) containing dipeptides: Met-X and X-Met, where X is: L-glycine (Gly), L-leucine (Leu), L-proline (Pro), and L-phenylalanine (Phe) are reported. Using pre-aggregated Ag colloid we obtained high-quality SERS spectra of these compounds spontaneously adsorbed on colloidal silver. Additionally, we measured Raman spectra (RS) of these heterodipeptides in a solid state as well as in acidic and basic solutions. The RS and SERS spectra of Met-X and X-Met presented in this work appear to be different. One of the most prominent and common features in the SERS spectra of all these dipeptides is a band in the 660-690 cm(-1) range that is due to the C-S stretching, v(CS), vibration of Met. This suggests that all the abovementioned compounds adsorb on the silver surface through a thioether atom. On the other hand, the SERS spectra of X-Met show clearly that not only the S atom but also the carboxylate group interact with the colloid surface as manifested by the enhancement of bands in the 920-930 and 1380-1396 cm(-1) regions. These bands are ascribed to the v(C-COO(-)) and v(sym)(COO(-)) vibrations, respectively. Additionally, a SERS spectrum of Phe-Met indicates that the interaction of the thioether atom, amine group, and aromatic side chain with the silver surface is favorable and may dictate the orientation and conformation of adsorbed peptide.     

Surface-enhanced Raman detection of melamine on silver-nanoparticle-decorated silver/carbon nanospheres: effect of metal ions

Silver/carbon (Ag/C) core-shell nanospheres synthesized by a hydrothermal method were used as templates for fabricating silver nanoparticle-decorated Ag/C (Ag/C/AgNps) nanospheres. The particle size of Ag nanoparticles can be tuned by varying the concentration of Ag precursor. Detection of melamine molecules at concentrations as low as 5.0×10(-8) M shows that the Ag/C/AgNps nanosphere is a good SERS-active substrate. The effect of heavy metal ions on the detection of melamine is also investigated. It was found that the SERS spectrum profile of melamine is very sensitive to the presence of heavy metal ions: the peak positions of the SERS bands exhibit some apparent change with the kind of metal ion, showing a blue or red shift compared with those in the SERS spectrum of melamine; the SERS signal intensity decrease with increasing the concentration of metal ion.     

Quantum dots trigger hot-start effects for pfu-based polymerase chain reaction

Hot-start (HS) effects were investigated in pfu-based polymerase chain reaction (PCR), when water-soluble CdTe quantum dots (QDs) were introduced in the PCR system. The HS effects were demonstrated by the higher amplicon yields and excellent suppression of non-specific amplification after pre-incubation of PCR mix with QDs between 35°C and 56°C. DNA targets were well amplified even after PCR mixture was pre-incubated 1?h at 50°C. Importantly, the effects of QDs nanoparticles could be reversed by increasing the pfu polymerase concentration, suggesting that there was an interaction between QDs and pfu DNA polymerase. Moreover, control experiment indicated that HS effect is not primarily due to the reduced pfu polymerase concentration resulted from the above interaction. Fluorescence correlation spectroscopy (FCS), a single molecule detection method, was used to investigate the possible mechanism of HS PCR with QDs. Preliminary FCS results suggested that CdTe QDs may directly interact with pfu DNA polymerase, rather than other components in the PCR system. Furthermore, results demonstrated that the interaction between QDs and pfu resulted in a reduction in pfu polymerase concentration. This study provided a good start to investigate potential implications of QDs in other key molecular biology techniques.     

Surface-enhanced raman scattering (SERS) detection of low concentrations of tryptophan amino acid in silver colloid

The surface-enhanced Raman scattering (SERS) spectrum of L-tryptophan has been studied in the concentration range 1.4 × 10(-8) to 5 × 10(-4) M. A borohydride-reduced silver colloid was employed as the nanoparticle enhancing agent and different electrolytes have been tested for activation of the colloid. The optimum conditions have been determined for achieving high sensitivity of detection. The experimental procedure developed, which includes the use of a composite electrolyte (a mixture of NaHCO(3) and NaCl) for colloid activation, results in very high enhancement of the Raman signal (up to 10(8)). This gives the possibility of studying SERS spectra of L-tryptophan at concentrations as low as 10(-8) M, which is several orders of magnitude lower than previously reported in the literature. The observed SERS spectra were very reproducible and were detectable 2 minutes after mixing, reaching maximum strength approximately 15 minutes after mixing. The spectral characteristics were stable over the entire period of observation. We have found that SERS spectra of tryptophan in silver colloid differ in several features at low (below ～10(-5) M) and at high (above ～10(-4) M) concentrations. The most important difference is the absence of the peak near 1000 cm(-1) at low concentrations, which is usually assigned to the indole ring breathing mode. The observed spectra allow us to suggest that at low concentrations Trp molecules bind to the surface through the indole ring, which remains flat on the surface. This is in contrast to the previously reported observation of SERS spectra from Trp performed at concentration levels above 10(-5) M.     

Detection of chemically induced DNA damage in layered films by catalytic square wave voltammetry using Ru(bpy)3(2+).

A sensor constructed by alternate layer-by-layer adsorption of PDDA cations and double-stranded (ds)-DNA on oxidized pyrolytic graphite electrodes was evaluated for detection of chemical damage to ds-DNA from known damage agent styrene oxide. Films made with PDDA ions of structure (PDDA/DNA)2 were approximately 6 nm thick and contained 0.23 microg of ds-DNA. Catalytic oxidation using 50 microM Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) and square wave voltammetry (SWV) provided more sensitive detection of DNA damage than direct SWV oxidation. The catalytic peaks increased linearly with time during incubations with styrene oxide, but only minor changes were detected during incubation with nonreactive toluene. For best sensitivity, the outer layer of the film must be ds-DNA, and analysis should be done at low salt concentration. Studies of DNA and polynucleotides in solutions and films suggested that oxidation of guanine and chemically damaged adenine in partly unraveled, damaged DNA were the most likely contributors to the catalytic peak.     

Freshwater toxic cyanobacteria induced DNA damage in apple (Malus pumila), rape (Brassica napus) and rice (Oryza sativa)

Cyanobacteria in freshwater ecosystems can present a harmful effect on growth and development of plants through irrigation with contaminated water. In this study, the effects of microcystins (MCs)-containing cyanobacteria extract (CE) on DNA damage of apple, rape and rice were investigated to explore the phytotoxic mechanism of MCs through DNA fragmentation and RAPD analysis. Determination of DNA fragmentation by fluorescent dye DAPI showed that significant DNA damage was observed in rice seedlings after exposure to CE while DNA fragmentation in rape seedlings and apple cultures did not differ significantly between treatment and control groups. Qualitative characterization of genomic DNA fragmentation by agarose gel electrophoresis supported the quantitative determination using DAPI. The main changes in RAPD profiles of rape seedlings following exposure of lower doses of CE were variation in band intensity for the primers F03 and S01, while higher doses of CE caused loss of normal bands and appearance of new bands except band intensity changes. The data presented here demonstrate that DNA damage in plants occurs following exposure of microcystins, and the polymorphic RAPDs may be used as an investigation tool for environmental toxicology and as a useful biomarker for the detection of genotoxic effects of microcystins on plants.     

Binding of aromatic isocyanides on gold nanoparticle surfaces investigated by surface-enhanced Raman scattering

The adsorption structure and binding of phenyl isocyanide (PNC), 2,6-dimethyl phenyl isocyanide (DMPNC), and benzyl isocyanide (BZI) on gold nanoparticle surfaces have been studied by means of surface-enhanced Raman scattering (SERS). PNC, DMPNC, and BZI have been found to adsorb on gold assuming a standing geometry with respect to the surfaces. The presence of the nu(CH) band in the SERS spectra denotes a vertical orientation of the phenyl ring of PNC, DMPNC, and BZI on Au. The lack of a substantial red shift and significant band broadening of the ring breathing modes implied that a direct ring pi orbital interaction with metal substrates should be quite low. For PNC, the band ascribed to the C-NC stretching vibration was found to almost disappear after adsorption on Au. On the other hand, the C-NC band remained quite strong for DMPNC after adsorption. This result suggests a rather bent angle of C-N[triple bond]C: for the nitrogen atom of the NC binding group on the surfaces, whereas a linear angle of C-N[triple bond]C: should be more favorable on gold surfaces due to an intramolecular steric hindrance of its two methyl groups. SERS of BZI on gold nanoparticles also supports a bent angle of :C[triple bond]N-CH2 for its nitrogen atom, suggesting a preference of sp3 (or sp2) hybridization for the nitrogen atom.     

Spectroscopic ultra-trace detection of nitroaromatic gas vapor on rationally designed two-dimensional nanoparticle cluster arrays

Nanoparticle cluster arrays (NCAs) are engineered two-dimensional plasmonic arrays that provide high signal enhancements for critical sensing applications using surface enhanced Raman spectroscopy (SERS). In this work we demonstrate that rationally designed NCAs are capable of detecting ultra-traces of 2,4-dinitrotoluene (DNT) vapor. NCAs functionalized with a thin film of an aqueous NaOH solution facilitated the detection of DNT vapor at a concentration of at least 10 ppt, even in the presence of an excess of potential interferents, including Diesel fuel, fertilizers, and pesticides. Both in the presence and in the absence of this complex background the SERS signal intensity of the NO(2) stretching mode showed a continuous, concentration dependent response over the entire monitored concentration range (10 ppt-100 ppb). The small size, superb sensitivity, and selectivity, as well as the fast response time of <5 min, make NCAs a valuable photonic sensor platform for ultra-trace nitroaromatic gas vapor detection with potential applications in landmine removal and homeland security.     

Antioxidant redox sensors based on DNA modified carbon screen-printed electrodes

Antioxidant redox sensors based on DNA modified carbon screen-printed electrodes were developed. The carbon ink was doped with TiO2 nanoparticles, onto which double-strand DNA was adsorbed. A redox mediator, namely, tris-2,2'-bipyridine ruthenium(II) [Ru(bpy)3(2+)] was electrooxidized on the electrode surface to subsequently oxidize both the adsorbed ds-DNA and the antioxidants in solution. The resulting oxidation damage of the adsorbed ds-DNA was then detected by square wave voltammetry in a second solution containing only Ru(bpy)3Cl2 at a low concentration (microM). A kinetic model was developed to study the protecting role of antioxidants in aqueous solutions. The electrochemical sensor has been applied to evaluate the redox antioxidant capacity of different molecules.     

Characterization of silane-modified immobilized gold colloids as a substrate for surface-enhanced Raman spectroscopy

Immobilized gold colloid particles coated with a C-18 alkylsilane layer have been characterized as a substrate for surface-enhanced Raman scattering (SERS) studies of adsorption onto hydrophobic surfaces. Atomic force microscopy images, optical extinction spectra, and SERS measurements are reported as a function of accumulation of gold colloid on glass. As the metal particles become increasingly aggregated on the surface, the SERS enhancement increases until the plasmon resonance shifts to wavelengths longer than the excitation laser. The gold colloid substrates are stable and exhibit reproducible SERS enhancement. When octadecyltrimethoxysilane is self-assembled over the gold, the metal surface is protected from exposure to solution-phase species, as evidenced by the inhibition of chemisorption of a disulfide reagent to the overcoated gold surface. The results show that interactions with gold can be blocked by a silane layer so as not to significantly influence physisorption of molecules at the C-18/solution interface. The SERS enhancement from these C-18-overcoated gold substrates is reproducible for different films prepared from the same colloidal suspension; the substrates are also stable with time and upon exposure to laser irradiation.     

Potential-dependent surface-enhanced Raman scattering from adsorbed thiocyanate for characterizing silver surfaces with improved reproducibility

Surface-enhanced Raman scattering (SERS) spectroelectrochemistry is used to characterize electrochemically roughened and highly polished polycrystalline silver SERS-active substrates. Changes in the nitrile stretching vibrational mode of adsorbed thiocyanate are used as an in situ spectroscopic probe: the potential dependence of band position (Stark tuning), shape, and scattering intensity of this mode are measured in order to investigate differences between SERS-active sites found on smooth and roughened electrode surfaces. Results obtained from thiocyanate adsorbed onto two different types of highly polished Ag surfaces (alumina and diamond polishing) show discrete populations of SERS-active adsorption sites that remain stable over a wide potential range. This behavior stands in contrast to that observed on electrochemically roughened surfaces, where very strong Stark tuning, large vibrational bandwidths, and irreversible loss of SERS enhancement upon negative potential excursions can be attributed to a diverse population of labile SERSactive sites that exhibit strong charge-transfer interactions with the adsorbate and large chemical SERS enhancement.     

Band analysis of temperature-dependent near-infrared spectra of oleic acid in the pure liquid state by the analytic geometric approach

The applicability of the band-stripping and complementary matching method has been demonstrated by the analysis of temperature-dependent near-infrared (NIR) absorption spectra in the 7500-6500 cm(-1) region of oleic acid (cis-9-octadecenoic acid) in the pure liquid state. This method is based on first derivative-second derivative pair (D1-D2) plots and a new concept called the complementary band, cBDi, created by subtracting all the rest of the bands, exclusive of the ith estimated band, eBDi, from an experimental spectrum. The degree of coincidence of both band shapes provides a suitable measure for the quality of fit for each individual component band. It has been confirmed from the present analysis of the NIR spectra of oleic acid measured over a temperature range of 16-79 degrees C that the change of the peak intensity of the component band at around 6915 cm(-1) due to the first overtone of an O-H stretching vibration of the monomer has two transition points around 35 and 55 degrees C. Moreover, the present study has provided new insight into the analysis of temperature-dependent spectral variations of oleic acid. Among the three temperature ranges, 16-35 degrees C, 35-55 degrees C, and 55-79 degrees C, in the first range the band near 6915 cm(-1) shows a slight increase and in the second range it has a linear intensity change with a slope of 0.002 a.u./degree C. In the third range, a rapid increase of the peak intensity is observed. This band exists even at 15 degrees C (just below the melting point) and shows a shift from 6910 to 6915 cm(-1) and a band narrowing from 85 to 80 cm(-1) (full width at half-height) over a temperature range of 16 to 79 degrees C. Furthermore, it has been found that there are two broad bands at around 6835 and 6778.     

Microfluidic Designing Microgels Containing Highly Concentrated Gold Nanoparticles for SERS Analysis of Complex Fluids

Surface‐enhanced Raman scattering (SERS) is one of the most promising methods to detect small molecules for point‐of‐care analysis as it is rapid, nondestructive, label‐free, and applicable for aqueous samples. Here, microgels containing highly concentrated yet evenly dispersed gold nanoparticles are designed to provide SERS substrates that simultaneously achieve contamination‐free metal surfaces and high signal enhancement and reproducibility. With capillary microfluidic devices, water‐in‐oil‐in‐water (W/O/W) double‐emulsion drops are prepared to contain gold nanoparticles and hydrogel precursors in innermost drop. Under hypertonic condition, water is selectively pumped out from the innermost drops. Therefore, gold nanoparticles are gently concentrated without forming aggregates, which are then captured by hydrogel matrix. The resulting microgels have a concentration of gold nanoparticles ≈30 times higher and show Raman intensity two orders of magnitude higher than those with no enrichment. In addition, even distribution of gold nanoparticles results in uniform Raman intensity, providing high signal reproducibility. Moreover, as the matrix of the microgel serves as a molecular filter, large adhesive proteins are rejected, which enables the direct detection of small molecules dissolved in the protein solution. It is believed that this advanced SERS platform is useful for in situ detection of toxic molecules in complex mixtures such as biological fluids, foods, and cosmetics.     

Role of the micro- and nanostructure in the performance of surface-enhanced Raman scattering substrates assembled from gold nanoparticles

Highly active and stable substrates for surface-enhanced Raman scattering (SERS) can be fabricated by using colloidal crystals to template gold nanoparticles into structured porous films. The structure-dependent performance of these SERS substrates was systematically characterized with cyanide in continuous flow microfluidic chambers. A matrix of experiments was designed to isolate the SERS contributions arising from nano- and microscale porosity, long-range ordering of the micropores, and the thickness of the nanoparticle layer. The SERS results were compared to the substrate structure observed by scanning electron microscopy (SEM) and optical microscopy to correlate substrate structure to SERS performance. The Raman peak intensity was consistently highest for nanoporous substrates with three-dimensionally ordered micropores, and decreases if the micropores are not ordered or not templated. Removing the nanoscale porosity by fusion of the nanoparticles (without removing the large micropores) leads to a drastic plunge in substrate performance. The peak intensity does not strongly correlate to the thickness of the nanoparticle films. The results make possible the efficient controlled fabrication of stable, reproducible, and highly active substrates for SERS based chemical sensors with continuous sampling.