Arumugam Sivanesan

Nanostructured silver–gold bimetallic SERS substrates for selective identification of bacteria in human blood

Surface-enhanced Raman spectroscopy (SERS) is a potentially important tool in the rapid and accurate detection of pathogenic bacteria in biological fluids. However, for diagnostic application of this technique, it is necessary to develop a highly sensitive, stable, biocompatible and reproducible SERS-active substrate. In this work, we have developed a silver-gold bimetallic SERS surface by a simple potentiostatic electrodeposition of a thin gold layer on an electrochemically roughened nanoscopic silver substrate. The resultant substrate was very stable under atmospheric conditions and exhibited the strong Raman enhancement with the high reproducibility of the recorded SERS spectra of bacteria (E. coli, S. enterica, S. epidermidis, and B. megaterium). The coating of the antibiotic over the SERS substrate selectively captured bacteria from blood samples and also increased the Raman signal in contrast to the bare surface. Finally, we have utilized the antibiotic-coated hybrid surface to selectively identify different pathogenic bacteria, namely E. coli, S. enterica and S. epidermidis from blood samples.

Amino Group Position Dependent Orientation of Self-Assembled Monomolecular Films of Tetraaminophthalocyanatocobalt(II) on Au Surfaces

Self-assembled monomolecular films of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4alpha-CoIITAPc) and 2,9,16,23-tetraaminophthalocyanatocobalt(II) (4beta-CoIITAPc) on Au surfaces were prepared by spontaneous adsorption from solution. These films were characterized by cyclic voltammetry and Raman spectroscopy. Both the surface coverage (Gamma) and intensity of the in-plane stretching bands obtained from Raman studies vary for these monomolecular films, indicating different orientations adopted by them on Au surfaces. The 4alpha-CoIITAPc-modified electrode exhibits an E1/2 of 0.35 V, while the 4beta-CoIITAPc-modified electrode exhibits an E1/2 of 0.19 V, corresponding to the CoII/CoIII redox couple in 0.1 M H2SO4. The Gamma estimated from the charge associated with the oxidation of Co(II) gives (2.62 +/- 0.10) x 10-11 mol cm-2 for 4alpha-CoIITAPc and (3.43 +/- 0.14) x 10-10 mol cm-2 for 4beta-CoIITAPc. In Raman spectral studies, the intensity ratio between in-plane phthalocyanine (Pc) stretching and the Au-N stretching was found to be 6.6 for 4beta-CoIITAPc, while it was 1.6 for 4alpha-CoIITAPc. The obtained lower Gamma and intensity ratio values suggest that 4alpha-CoIITAPc adopts nearly a parallel orientation on the Au surface, while the higher Gamma and intensity ratio values suggest that 4beta-CoIITAPc adopts a perpendicular orientation. The electrochemical reduction of dioxygen was carried out using these differently oriented Pcs in phosphate buffer solution (pH 7.2). Both the Pcs catalyze the reduction of dioxygen; however, the 4alpha-CoIITAPc-modified electrode greatly reduces the dioxygen reduction overpotential compared to 4beta-CoIITAPc-modified and bare Au electrodes.

Potential-Dependent Surface-Enhanced Resonance Raman Spectroscopy at Nanostructured TiO2: A Case Study on Cytochrome b5

Nanostructured titanium dioxide (TiO2 ) electrodes, prepared by anodization of titanium, are employed to probe the electron-transfer process of cytochrome b5 (cyt b5 ) by surface-enhanced resonance Raman (SERR) spectroscopy. Concomitant with the increased nanoscopic surface roughness of TiO2 , achieved by raising the anodization voltage from 10 to 20 V, the enhancement factor increases from 2.4 to 8.6, which is rationalized by calculations of the electric field enhancement. Cyt b5 is immobilized on TiO2 under preservation of its native structure but it displays a non-ideal redox behavior due to the limited conductivity of the electrode material. The electron-transfer efficiency which depends on the crystalline phase of TiO2 has to be improved by appropriate doping for applications in bioelectrochemistry.

Complementary surface-enhanced resonance Raman spectroscopic biodetection of mixed protein solutions by chitosan- and silica-coated plasmon-tuned silver nanoparticles.

Silver nanoparticles with identical plasmonic properties but different surface functionalities are synthesized and tested as chemically selective surface-enhanced resonance Raman (SERR) amplifiers in a two-component protein solution. The surface plasmon resonances of the particles are tuned to 413 nm to match the molecular resonance of protein heme cofactors. Biocompatible functionalization of the nanoparticles with a thin film of chitosan yields selective SERR enhancement of the anionic protein cytochrome b(5), whereas functionalization with SiO(2) amplifies only the spectra of the cationic protein cytochrome c. As a result, subsequent addition of the two differently functionalized particles yields complementary information on the same mixed protein sample solution. Finally, the applicability of chitosan-coated Ag nanoparticles for protein separation was tested by in situ resonance Raman spectroscopy.

Electrochemical current rectification–a novel signal amplification strategy for highly sensitive and selective aptamer-based biosensor

Electrochemical aptamer-based (E-AB) sensors represent an emerging class of recently developed sensors. However, numerous of these sensors are limited by a low surface density of electrode-bound redox-oligonucleotides which are used as probe. Here we propose to use the concept of electrochemical current rectification (ECR) for the enhancement of the redox signal of E-AB sensors. Commonly, the probe-DNA performs a change in conformation during target binding and enables a nonrecurring charge transfer between redox-tag and electrode. In our system, the redox-tag of the probe-DNA is continuously replenished by solution-phase redox molecules. A unidirectional electron transfer from electrode via surface-linked redox-tag to the solution-phase redox molecules arises that efficiently amplifies the current response. Using this robust and straight-forward strategy, the developed sensor showed a substantial signal amplification and consequently improved sensitivity with a calculated detection limit of 114nM for ATP, which was improved by one order of magnitude compared with the amplification-free detection and superior to other previous detection results using enzymes or nanomaterials-based signal amplification. To the best of our knowledge, this is the first demonstration of an aptamer-based electrochemical biosensor involving electrochemical rectification, which can be presumably transferred to other biomedical sensor systems.

Fabrication of optochemical and electrochemical sensors using thin films of porphyrin and phthalocyanine derivatives

AbstractThis paper describes the fabrication of thin films of porphyrin and metallophthalocyanine derivatives on different substrates for the optochemical detection of HCl gas and electrochemical determination of L-cysteine (CySH). Solid state gas sensor for HCl gas was fabricated by coating meso-substituted porphyrin derivatives on glass slide and examined optochemical sensing of HCl gas. The concentration of gaseous HCl was monitored from the changes in the absorbance of Soret band. Among the different porphyrin derivatives, meso- tetramesitylporphyrin (MTMP) coated film showed excellent sensitivity towards HCl and achieved a detection limit of 0.03ppm HCl. Further, we have studied the self-assembly of 1,8,15,22-tetraaminometallophthalocyanine (4α-MTAPc; M = Co and Ni) from DMF on GC electrode. The CVs for the self-assembled monolayers (SAMs) of 4α-CoIITAPc and 4α-NiIITAPc show two pairs of well-defined redox couple corresponding to metal and ring. Using the 4α-CoIITAPc SAM modified electrode, sensitive and selective detection of L-cysteine was demonstrated. Further, the SAM modified electrode also successfully separates the oxidation potentials of AA and CySH with a peak separation of 320mV. Graphical AbstractThin films of meso-tetramesitylporphyrin on glass slide and 1,8,15,22-tetraaminometallophthalocyanine on GC electrode were fabricated for the optochemical detection of HCl gas and electrochemical determination of L-cysteine.

Towards interference free HPLC-SERS for the trace analysis of drug metabolites in biological fluids

&NA; Sofosbuvir metabolite, 2′‐deoxy‐2′‐fluoro‐2′‐C‐methyluridine (PSI‐6206) was studied for the first time by surface enhanced Raman spectroscopy (SERS) using the paper‐based SERS substrate. The quantification limit of PSI‐6206 by SERS was found to be 13 ng L−1 (R2 value = 0.959, RSD = 5.23%). For the structural and quantitative analysis of PSI‐6206 in blood plasma, an interference‐free HPLC‐SERS method was developed and compared to HPLC‐DAD and HPLC–MS methods. The SERS quantification of the drug by the paper substrate was 4 orders of magnitude more sensitive than that by the diode array detector. In addition, the SERS detection provided unique structural identification of the drug in blood plasma, similar to Mass spectroscopy detector. Due to the disposable nature of the SERS substrate, the new method does not suffer from the known “memory effect” which is known to lead to false positive identification in traditional HPLC‐SERS methods. Therefore, the presented HPLC‐paper SERS platform holds great potential for the sensitive and cost effective determination of drugs and their metabolites in biological fluids. Graphical abstract Figure. No caption available. HighlightsSofosbuvir metabolite was studied for the first time by Raman spectroscopy.Cheap and disposable paper substrate was utilized for the determination of PSI‐6206 by HPLC‐SERS.The use of disposable substrate eliminated the memory effect problem in the HPLC‐SERS.The SERS LOQ of PSI‐6206 was 13 ng L‐1 (R2 = 0.959, RSD = 5.23%), 4 orders of magnitude less than HPLC‐DAD.The HPLC‐SERS method provided unique structural identification of PSI‐6206 similar to HPLC‐MS

Plasmon-tuned silver colloids for SERRS analysis of methemoglobin with preserved nativity.

Optically tuned silver nanoparticles (AgNPs) functionalized with ω-mercaptoalkanoic acids are synthesized and used as a signal amplifier for the surface-enhanced resonance Raman scattering (SERRS) study of heme cofactor in methemoglobin (metHb). Even though both mercaptopropionic acid (MPA)- and mercaptononanoic acid (MNA)-functionalized AgNPs exemplify vastly enhanced SERRS signal of metHb, MNA-AgNPs amplify the SERRS signal amid preservation of the nativity of the heme pocket, unlike MPA-AgNPs. The electrostatic interaction between MNA-AgNPs and metHb leads to instant signal enhancement with a Raman enhancement factor (EF(SERS)) of 4.2 × 10(3). Additionally, a Langmuir adsorption isotherm has been employed for the adsorption of metHb on the MNA-AgNP surface, which provides the real surface coverage and equilibrium constant (K) of metHb as 139 nM and 3.6 × 10(8) M(-1), respectively. The lowest detection limit of 10 nM for metHb has been demonstrated using MNA-AgNPs besides retaining the nativity of the heme pocket.

Regenerative silver nanoparticles for SERRS investigation of metmyoglobin with conserved heme pocket

Shell isolated silver nanoparticles with an ultrathin silica layer (Ag@SiO2NPs) are used as a surface-enhanced resonance Raman scattering (SERRS) substrate for probing metmyoglobin (metMb) in aqueous solution. The ultrathin silica layer protects metMb from reaching the bare silver surface and conserves the heme pocket during SERRS analysis with a Raman enhancement factor (EFSERS) of 4.78 × 104. In spite of the good SERRS enhancement, the interaction between the protein and Ag@SiO2NPs is weak enough to separate them by centrifugation in such a way that both are regenerated in their original form and can be reused. Using Ag@SiO2NPs as the SERRS substrate, the lowest detection limit of 2 nM was achieved for metMb whilst conserving the native structure of the heme centre.

Rapid detection of mercury contamination in water by surface enhanced Raman spectroscopy

Mercury (Hg) is a potent neurotoxin in fish, wildlife, and humans. The detection of Hg( II ) ions in water therefore requires accurate, ultra-sensitive, rapid and cost effective analytical methods. We present a novel nanosensor for the field detection of Hg( II ) in water by surface enhanced Raman spectroscopy (SERS). In the new SERS nanosensor, aminodibenzo-18-crown-6 (ADB18C6) was coupled with mercaptopropionic acid and the resultant crown ether derivative (TCE) was self-assembled as a recognition surface layer for Hg( II ) onto the surface of a nanostructured gold substrate. The coordination of Hg( II ) to the oxygen atoms of TCE led to the spontaneous binding of the metal ion into the cavity of the crown ether layer. This caused the intensity of the Raman band at 1501 cm −1 for the crown ether to increase with the concentration of Hg( II ) in the range of 1 × 10 −11 M to 1 × 10 −6 M. Complexation between TCE and Hg( II ) was further confirmed by UV-visible spectrometry, spectrofluorimetry and electrochemistry. The method was successfully applied to the determination of Hg( II ) in tap water using a handheld Raman spectrometer and it demonstrated high selectivity towards Hg( II ) in the presence of Pb( II ) and Cd( II ). This eliminated the need for extensive sample preparation and extraction procedures prior to the analysis. The limit of Hg( II ) quantification by the new SERS nanosensor and the limit of detection were 1000 fold below the EPA and WHO defined levels for Hg( II ) ions in water.