Photoelectrochemical studies of DNA-tagged biomolecules on Au and Au/Ni/Au multilayer nanowires
© Swaminathan et al; licensee Springer. 2011
Received: 5 July 2011
Accepted: 30 September 2011
Published: 30 September 2011
The use of nanowires (NWs) for labeling, sensing, and sorting is the basis of detecting biomolecules attached on NWs by optical and magnetic properties. In spite of many advantages, the use of biomolecules-attached NWs sensing by photoelectrochemical (PEC) study is almost non-existent. In this article, the PEC study of dye-attached single-stranded DNA on Au NWs and Au-Ni-Au multilayer NWs prepared by pulse electrodeposition are investigated. Owing to quantum-quenching effect, the multilayer Au NWs exhibit low optical absorbance when compared with Au NWs. The tagged Au NWs show good fluorescence (emission) at 570 nm, indicating significant improvement in the reflectivity. Optimum results obtained for tagged Au NWs attached on functionalized carbon electrodes and its PEC behavior is also presented. A twofold enhancement in photocurrent is observed with an average dark current of 10 μA for Au NWs coated on functionalized sensing electrode. The importance of these PEC and optical studies provides an inexpensive and facile processing platform for Au NWs that may be suitable for biolabeling applications.
Gold (Au) nanostructures have paved the way to map out a novel platform for designing nano biobarcode for a wide range of biosensing applications . Au nanomaterials, such as nanoparticles, nanowires (NWs), and nanorods, are the widely studied materials which have great demand in the scientific community [2, 3]. Interestingly, they offer a number of properties that make them suitable for use in biological applications, such as biosensing , biosorting , and biolabelling . The structure and composition in multilayered gold NWs will escalate the development of bio-nanotechnology when compared with nanoparticles . In particular, 1D Au nanostructures have a strong optical property that can be tuned by controlling the wire length and diameter of the NWs and multilayer NWs . Moreover, the optical absorption coefficient of gold NWs is much higher than those of gold nanoparticles [9–11]. The fabricated NWs are tagged with various DNA libraries, antibodies, or antigens that can be used for sensing or labeling at a time of different biological assays through direct chemical reactions .
A suitable synthesis technique is needed to control the shape and size of the NWs to improve the biocompatibility for biosensing applications. The most direct approach of controlled synthesis of NWs is produced by electrochemical routes . High aspect ratio NWs have more intense reflection and scattering properties; dominated by the polarization-dependent plasmon resonance between the metallic layers rather than by the bulk metallic reflectance . The identification of tagged biomolecules on the surface of nanomaterials can be encoded and easily read out through optical microscope . The optical properties of Au or Au stripes nanostructures , optical quenching , and the NW aggregation  have widely been reported, but the understanding of surface plasmon for multilayer NWs is still to be explored. Hence, it is important to study the shape of multilayer NWs that affects the surface plasmon [18, 19], which is the key area to tune the optical properties of biobarcode in multiplex biolabeling applications.
Photoelectrochemical (PEC) measurements have been well exploited for photovoltaic applications, but the literature is scarce on the detection of biomolecules using this approach. PEC is simple and offers an alternative method of detecting biomolecules through molecular binding on a working electrode by electrochemical route. Thus, we study the PCE properties of tagged Au nanostructures. In this article, we describe the effect of surface plasmon and the variation of luminescence properties on shape-controlled Au nanostructures that tagged with thiolated cy3-dye attached on DNA. We also study the PEC properties of dye with DNA-tagged Au and multilayer NWs coated on functionalized carbon electrode.
Field emission scanning electron microscope (JSM-6335 FESEM) was employed to study the morphology of Au NWs and multilayer NWs. A bright-field reflectance images were acquired using an inverted microscope (Olympus BX 51,175 W ozone-free He lamp), equipped with a color digital video camera (Sony Exwave HAD-12 megapixel). All reflectance images were taken at 540 nm, which is the wavelength that gives the optimum reflectance area of the Au NWs. A confocal Raman system (WITEC CRM-200) with a processing time of 0.5 s was used to measure the photoluminescence (PL) spectrum of Au NWs and multilayer NWs.
Au NWs and multilayer NWs (150 μL) were first incubated with dNTP (0.2 μL, 10 mM) for 15 min. Then, 300 μL buffer containing NaCl (50 mM) and sodium phosphate (5 mM) was added into the mixture. The volume was reduced to 150 μL by vacuum centrifugation over 4-5 h at 45°C to gradually increase salt concentration which is critical to maintain a stable colloid solution. Then, thiol-DNA was introduced in, followed by heating at 55°C for 3 h. Subsequently, the particles were washed through centrifugation to remove unbound oligonucleotides. Fluorescence of the tagged DNA on gold was accomplished by means of a fluorophores-Cy3-dye (green emission) which was covalently attached to the oligonucleotides used in the sequence of (5'-3'): (5ThioMC6-D/TTT TTT TTT TCC CTA ACC CTA ACC CTA ACC CTT/3Cy3Sp).
PEC measurements were carried out using a three-electrode electrochemical cell and a light source of 200lumens LED (Fenix PP). The resistance of the screen printed electrode was 50 ± 10 Ω. To improve the conductivity of the electrode, 2 μL of BMIM-PF6 ionic liquid was coated on the screen-printed carbon surface. The significance of the ionic liquid is that it can improve the conductivity, resulting in low ohmic losses and high rate of mass transfer. Au NWs were then drop-cast on the functionalized screen-printed electrodes. Before electrochemical detection of biomolecules, the dried electrodes were rinsed with pH of 7.4 PBS for further analysis. A three-electrode setup consisting of the functionalized electrode as photo cathode, SCE as reference, and the platinum electrode as anode were used to measure photocurrent upon light irradiation. 20 mL of PBS was used as electrolyte; photocurrent was then recorded as a function of light irradiation. PEC measurements were taken for raw electrode, dark, and light current measurements for the surface-modified photo cathode.
Results and discussion
The Au NWs were dispersed in different solvents: water and IPA. Owing to different refractive index of the solvents, the reflected intensity of the plasmon band varies significantly with respect to the solvents. Hence, the optical absorbance of Au NWs in IPA is stronger than that in water. It is anticipated that the surface plasmon was dependent on the shape of the particles, the nature of the dispersing solvent, and the aggregation of nanomaterials . Therefore, the maximum optical absorption was observed for Au NWs, particularly dispersed in IPA (Figure 3a). The absorption behavior was different, even though similar size of templates was used for the synthesis of Au NWs and multilayer NWs. The optical absorbance was lower in multilayer NWs because of the amount of wire aggregation and the force of attraction between the wires as Ni is a ferromagnetic material. An enhanced intensity of the plasmon with less aggregation can be obtained when suitable dispersing solvent was used. In Au NWs, there is a minor shift in the absorbance band toward longer wavelengths at 660 and 770 nm, which can be attributed to shape the NWs and coupling between the Au NWs aggregation .
In summary, Au NWs and multilayer NWs have successfully been prepared using electrodeposition technique and tagged with cy3-dye with DNA biomolecules. The optical and PEC properties have been investigated. Owing to surface plasmon resonance, Au NW showed maximum optical absorbance and PL. The PEC characteristics of Au NWs exhibited a photocurrent of 35 μA, which is because of the movement of charge carriers in the dye and their excitation to conduction band, which increase drastically the photocurrent to two orders of magnitude from initial dark current values. This study provides a platform in the area of biosensing which can be accomplished by PEC measurements.
This study was supported in part by the ASTAR SERC grant (082 101 0015) and the NRF-CRP program (Multifunctional Spintronic Materials and Devices). We thank Mr. Bin Yan of SPMS (NTU) for his assistance in laser-induced PL measurements.
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