| In this paper, two-dimensional ordered micro/nano-scale non-metallic (SiO2) andmetal (gold, silver) spherical cavity array have been fabricated through electrochemicaland electroless deposition method using close-packed monodisperse polystyrene (PS)spheres as templates. The morphology of the as-prepared cavity array can be modulatedby changing the size of microspheres and the deposition conditions. On this basis, anovel H2O2electrochemical biosensor has been constructed. Next, we have developed akind of substrates for surface-enhanced Raman scattering (SERS) based on gold andsilver cavity arrays, which are further applied in the research of chemical enhancementof SERS, and the SERS detection of polycyclic aromatic hydrocarbons (PAHs) andprotein.Major innovative research results are as follows:1. A close-packed two-dimensional ordered PS template are first constructed onthe interface of water and gas via self-assembly method, the ordered PS template can beeasily transferred to the surface of an indium-doped tin oxide (ITO) glass slide or a goldcoated ITO glass slide, which are used as working electrode for the electrodeposition. Inthe case of silica cavity fabrication, OH－can be generated near the electrode surface byelectrochemical reduction of H2O2, promoting the hydrolysis and condensation oftetramethoxysilane at the interstices among the templates, thereby forming a highlyordered silica array structure. The shape of the silica cavity array can be controlled bychanging the electrochemical reaction conditions. To obtain regular structure of themetal (gold, silver) cavity array, gold thin film coated ITO glass slides are used asworking electrodes, multi-current pulse plating with the first pulse to a current densityof30mA cm-2for100ms followed by a train of pulses of5mA cm-2for60msseparated by a rest time of1s (zero current) is chosen as the proper deposition method.Similarly, we also can control the morphology of the gold/silver cavity array bychanging the electrochemical reaction conditions (changing small pulse repetition frequency), which is critically important of these cavity array in further SERSapplication.2. We have developed a novel hydrogen peroxide biosensor based on the silicacavity array modified ITO electrode. A two-dimensional array of hemispherical silicacavities, opened at top and bottom, are fabricated on the surface of ITO electrodes using2μm diameter PS as the template, and followed by the electrochemical deposition ofgold nanoparticles (GNPs) at the bottom of the cavities. A monolayer of4-mercaptobenzoic acid (MBA) is then attached to the GNPs through S-Au bond, andacts as a mediator to covalently immobilize MP-11. Due to the resistance gradient of thesilica cavity structure, the silica cavity exhibits a confinement effect on theelectrochemical reactions. The electrode exhibits a fast transfer rate for electronsbetween the MP-11molecules, via the GNPs, and the ITO electrode. Furthermore, themodified electrode shows a high electrocatalytic activity towards the reduction ofhydrogen peroxide and a good analytical performance for the amperometric detection ofH2O2with a linear range from2×10–6to6×10–4M，the limit of detection is6×10-7M.The good reproducibility and long-term stability of this novel electrode not only offeran opportunity for the detection of H2O2in low concentration, but also provide aplatform to construct various biosensors based on many other enzymes.3. Based on the surface charge characteristic of the silica cavities, we haveproduced a research on chemical enhancement mechanism of SERS. In this section,silver nanoparticles (AgNPs) are assembled onto the bottom of closed-packed silicacavity, and then modified with a monolayer of p-aminothiophenol (PATP). Chargetransfer between the adsorbed PATP molecules and the silver nanoparticles has beenstudied using SERS with514nm,633nm,785nm and1064nm excitation. Using theconcept of degree of charge transfer, we have directly observed the additional chemicalenhancement without a deliberate distinction between electromagnetic (EM)enhancement and chemical enhancement. We demonstrate that optimization of theenvironmental condition can result in an alteration of the local Fermi level of the metalnanoparticles, thus leading to better energy matching between the energy levels of theinterconnecting molecules and the Fermi level of the metal. In this paper，the negativecharged silica cavity can alter the dipolar orientation of the AgNPs inside it, enlargingthe electron density at the sites where probe molecules adsorbed. The interface dipole with its positive pole towards the metal decreases the Fermi energy of the metal, makingit favorable for the charge transfer from metal to molecule, and in turn, a greater CTenhancement can be achieved.4. We have developed a kind of novel SERS-active substrates with remarkableenhancement and high reproducibility for trace polycyclic aromatic hydrocarbon (PAHs)detection and protein detection. A series of gold/silver cavity arrays with controllableapertures and cavity depth have been prepared using a multi-current pulse method. Byadjusting the repeat times of the low current pulse, we can achieve the modulation of thesubstrate localized surface plasmon resonance (LSPR) from visible region to nearinfrared region. This allows us to optimize the SERS substrate according to theexcitation lines (514nm,633nm,785nm) and probe molecules, and to select the mostappropriate SERS substrates for SERS detection of small molecules (such as PAHs) andbiological macromolecular (such as proteins).(1) Application in the detection of PAHs. PAHs are high hydrophobicity with lowaffinity for the metallic surfaces and present no specific functional group (such as thiolor amino) that could act as an anchor to the surface of the substrate, therefore, it isdifficult to detect PAHs with a general SERS substrate. In this paper, we modify thesilver cavity array with a monolayer of1,10-decanedithiol via self-assembly method.The functions of the1,10-decanedithiol lie in two aspects: one is to effectively adsorbPAHs molecules to the surface of metallic substrates according tosimilarity-intermiscibility; the other is to introduce silver nanoparticles (AgNPs) intothe cavities by a S-Ag bond to form a AgNPs/PAHs/silver cavity array sandwichstructure. The measured SERS spectra allow easy distinction of anthracene and pyrene;the two PAH compounds can be detected over a wide concentration range and thedetection limit of anthracene and pyrene were8and40nM, respectively. The resultsdemonstrate that the new SERS substrate is suitable for the quantitative identification ofnon-polar organic pollutants like PAHs.(2) Application in protein detection. Proteins are one of the most versatile groupsof molecules with vital functional roles in living systems. The detection of proteins iscritically important in biomedical research, diagnosis, proteomics and systems biology.In this paper, we employ a silver cavity array as a sensitive enhancement substratetogether with silver staining technique for the qualitative and quantitative detection ofproteins through Raman spectroscopy. The proposed method allows direct and rapid detection of human IgG based on the immune-recognition of the corresponding labeledantibody, atto610-labeled biotin/avidin recognition, as well as label-free proteins. SERSspectra of these proteins and Raman labels on the proposed substrates show both highsensitivity and reproducibility, which can be attributed to the following aspects. First,the bowl-shaped silver cavity (BSSC) array structure acts as an optical trap to harvestthe light, which is then focused in the presence of the silver nanoparticles due to theconcave focusing effect. Besides, such a structure allows the entire immunocomplexesin the coupled electromagnetic field of silver cavity and silver nanoparticles, thusgenerating a significantly enhanced Raman signal. Second, the upper silvernanoparticles show no effect on the specific recognition reaction between antigen andcorresponding antibody and, the direct contaction between silver nanoparticles and thefluorescent dye molecules can enhance the Raman signal while quenching thefluorescence signal. The results demonstrate that the proposed the approach has thepotential for qualitative and quantitative detection of biomolecules. |
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