This application is related to and claims the benefit under 35 U.S.C. 119 to the Korean patent application bearing serial number 10-2009-0027800, entitled “Microsphere Having Hot Spots and Method For Identifying Chemicals Through Surface Enhanced Raman Scattering Using the Same”, filed Mar. 31, 2009. The content of the Korean patent application 10-2009-0027800 is incorporated herein in its entirety by reference.
The present invention relates to the microsphere whose surface is covered with hot spots and the method for identifying chemicals through surface enhanced Raman scattering (SERS) using the same.
Surface-Enhanced Raman Scattering (SERS) has been a subject of intensive research since it suggests many useful applications such as biological sensing and trace analysis (Haynes, C. L.; McFarland, A. D.; Van Duyne, R. P. Anal. Chem. 2005, 77, 338A-346A). The unique signal enhancement of SERS reportedly allows the detection of analytes even at the single-molecule level. The huge enhancement in SERS process is known to occur at the so-called “hot spots”, which originate from either interstitial sites or the surface of nano-sized materials. Moskovits et al. proposed a simple strategy for creating SERS hot spots among closely spaced nanowires and showed that the enhancement is a function of interstitial distances (Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200-2201). They also demonstrated a chemically patterned SERS-active system where the hot spots are easily found and analyzed (Braun, G.; Pavel, I.; Morrill, A. R.; Seferos, D. S.; Bazan, G. C.; Reich, N. O.; Moskovits, M. J. Am. Chem. Soc. 2007, 129, 7760-7761).
Electrochemically roughened metal surfaces and colloidal nanoparticles are traditionally employed as SERS substrates (Haynes, C. L.; McFarland, A. D.; Van Duyne, R. P. Anal. Chem. 2005, 77, 338A-346A). Recently more stable and controllable SERS-active substrates are reported including nanoparticle arrays fabricated with nanosphere lithography (Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. C 2003, 107, 7426-7433), nanowire bundles (Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200-2201), and metal surfaces by templated electrodeposition (Abdelsalam, M. E.; Bartlett, P. N.; Baumberg, J. J.; Cintra, S.; Kelf, T. A.; Russell, A. E. Electrochem. Commun. 2005, 7, 740-744). On the other hand, tip-enhanced Raman scattering (TERS) has received extensive attention since it can provide a high spatial resolution as well as specific chemical information (Baldelli, S. Chemphyschem 2008, 9, 2291-2298). However, TERS at the present stage requires highly sophisticated setups and suffers from low signal enhancement, poor reproducibility, and difficulty in working in aqueous media. Halas et al. proposed a somewhat different approach, where a nanoshell geometry consisting of a dielectric core with a thin gold coating is utilized as SERS-active substrates (Oldenburg, S. J.; Westcott, S. L.; Averitt, R. D.; Halas, N. J. J. Chem. Phys. 1999, 111, 4729-4735). The use of an individual nanoshell as a SERS probe may provide a simple and efficient method to identify the molecules on a SERS-inactive substrate, which has been one of the challenging issues in current surface analysis. Another valuable application of an individual SERS-active particle in modern bioanalysis is a multiplex assay based on the respectively bar-coded microspheres with combination of SERS tags flowing in micro fluidic channels (Jun, B. H.; Kim, J. H.; Park, H.; Kim, J. S.; Yu, K. N.; Lee, S. M.; Choi, H.; Kwak, S. Y.; Kim, Y. K.; Jeong, D. H.; Cho, M. H.; Lee, Y. S. J. Comb. Chem. 2007, 9, 237-244; Jin, R. C.; Cao, Y. C.; Thaxton, C. S.; Mirkin, C. A. Small 2006, 2, 375-380). This strategy increasingly attracts keen attention from the viewpoint of acquisition more information in a smaller volume of sample within a shorter time. However, SERS signals from an individual nanoshell are too weak to be utilized for both surface probing and multiplex analysis. Moreover, the nanoshells are too small to be recognized by a conventional optical microscopy and thus hard to be individually addressed and manipulated.
According to embodiments of the present invention, it is reported that the microsphere with SERS-active Au shell, SERS signals from which are maximized. The microsphere proposed in the present invention has the size dimension that can be recognized by a conventional optical microscope. The microsphere proposed in the present invention suggests opportunities not only for sensitive surface probing and imaging of an organic molecule monolayer but also for multiplex assay based on bar coded microspheres using SERS techniques.
According to an embodiment of the present invention, an exemplary microsphere that has hot spots is provided and includes a microsphere and metal networks as a shell which covers the surface of the microsphere. Nano-sized pores are distributed randomly on the surface or in the interstitial space of the metal networks. The metal networks are formed with nanoparticles of SERS-active metals. For example, the metal networks are formed with SERS-active metals such as gold, silver, platinum and copper.
According to yet another embodiment of the present invention, another exemplary microsphere having hot spots is provided that has nano-size pores which are distributed randomly on the surface or in the interstitial space of the metal networks, where the size of pores is in the range of 1-30 nm, more desirably between 1-20 nm, and most desirably between 1-10 nm.
According to yet another embodiment of the present invention, a method for identifying chemicals through Surface-Enhanced Raman Scattering (SERS) is provided using the microsphere, where the method includes the following process of adsorption of analyte on the microsphere, generation of SERS signals from the microsphere on which the analyte is adsorbed, collection of the SERS signals, analysis of the SERS signals, and identification of the analyte.
According to embodiments of the present invention, the microsphere having hot spots and the SERS method using the same, provide the solution which can overcome the weakness of existing SERS method using nanoparticles. The microsphere having hot spots provides an advantage of easy manipulation whereas existing SERS methods using nanoparticles have difficulty in handling nanoparticles.
In addition, the microsphere having hot spots provides another advantage of being monitored through an optical microscope. Using a typical micropipette or optical tweezers, a single microsphere may be precisely placed on the targeted spot, trans-located to another spot and removed from the surface under the naked eye monitoring through an optical microscope.
Since the SERS activity of the microsphere having hot spots is stable in water, this system can be utilized for in situ surface chemical probing in aqueous phase, which is especially important for most of electro catalysis studies.
The microsphere having hot spots offers new opportunities for SERS-based probing techniques to a wide range of valuable applications such as a universal and reliable way of chemical identification with a high spatial resolution on surfaces, in vivo chemical or biological monitoring on the membrane of a living cell like neuron or stem cell, and high throughput decoding of microsphere suspension arrays in micro fluidic systems.
According to embodiments of the present invention, the microsphere having hot spots includes A) a microsphere and B) metal networks as a shell which covers the surface of the microsphere, and nano-sized pores are distributed randomly on the surface or in the interstitial space of the above metal networks.
The above microspheres can be synthesized with various materials and can be even synthesized with SERS-inactive materials. For example, microspheres include a polymer sphere of micro size, a metal particle of micro size, a silica particle of micro size, and a magnetic polymer sphere of micro size. In the embodiment of present invention, micro size means 1-1000 μm, more desirably 1-100 nm, and most desirably 1-30 nm.
The shell of metal networks is formed on the surface of the above microsphere, where the above metal networks are made of nanoparticles of SERS-active metals. Desirably, the above metal networks are made of nanoparticles of SERS-active metal such as gold, silver, platinum and copper. Nano-sized pores are distributed randomly on the surface or in the interstitial space of the above metal networks and they create hot spots. For example, the above metal networks can be formed by nanoparticles of 3-30 nm.
The size of pores which are distributed randomly on the surface or in the interstitial space of the above metal networks is 1-30 nm, more desirably 1-20 nm, and most desirably 1-10 nm.
According to embodiments of the present invention, the microsphere having hot spots is useful for identification of chemicals by Surface-Enhanced Raman Scattering (SERS). The above method includes adsorption of analyte on the microsphere, generation of SERS signals from the microsphere on which the analyte is adsorbed, collection of the SERS signals, analysis of the SERS signals, and identification of the above analyte.
In the following, examples further illustrate the structures and methods of this invention.
Microspheres with metal network structures (MS-MeNet) can be prepared by modification of established methods. Microspheres with Au network structures (MS-AuNet) are prepared based on procedures for Au nanoshells. Amine-terminated polystyrene spheres (d=10.8 μm) are decorated with Au nanoparticles (AuNPs) to provide nucleation sites, which are subsequently grown by electroless plating. The plating steps are repeated to gradually increase the size of Au nanoparticles on the microsphere surface.
The SERS activity from MS-AuNets modified with 4-nitrobenzenethiol (NBT) as a function of the number of repetitive electroless plating is examined and the result is summarized in
a) also shows the cross sectional TEM images of MS-AuNets, which rationalize the dependence of SERS activity on the number of plating steps. The inter-particle distances among the AuNPs of the seed layer on a polystyrene sphere may be too large to create effective hot spots. A similar result was previously reported that SERS activities from individual AuNPs fixed on a flat silicon wafer surface are negligible until the inter-particle distance becomes sufficiently small and the inter-particle coupling progresses to some degree. As the number of gold plating increases to 5-10 times, gold nanoparticles grow into larger particles of about 20 nm and a part of them merge each other. The enhancement of SERS signals implies that the nanopores act as SERS-active hot spots. In other words, it is implied that the increase of gold plating to 5-10 times make the nanopores on the surface or in the interstitial place of gold networks and they create hot spots. The maximum enhancement of SERS peaks is shown when gold plating is repeated 10 times. As identified in the TEM image (the inset of gold plating for 10 times and
The MS-AuNets modified with NBT shows stable and reproducible SERS activities when re-dispersed in pure ethanol or water, where the SERS signals exhibit the same dependence on the extent of plating. NBT-modified MS-AuNets on a glass slide are recognized through a conventional optical microscope (
A bare single MS-AuNet placed on a monolayer of NBT on Au substrates also induces a SERS-active environment. The 10-times plated bare MS-AuNet provides not only a nano-gap effect but also hot spots on its own surface. The enhancement factor (EF) of SERS activity is estimated to be 2.5×105. It produces significantly larger signals than those from a 30-times plated MS-AuNet with a completed Au layer providing only nano-gap effect as shown in
Microspheres having silver hot spots are prepared as another example for microspheres with metal network structures (MS-MeNet).
As shown in the above embodiments, the Au shell on a polymer microsphere may be tuned to achieve a strong SERS-active platform so that the molecules on entire or even only a part of a single MS-AuNet surface produce their own fingerprint SERS spectra. The proposed MS-AuNets can be identified and individually addressed using a conventional optical microscope. A single MS-AuNet may be used to act as a probe to obtain Raman spectra of monolayered molecules on Au and Pt surfaces by signal acquisition for about 1 ms, which is useful for decoding the microspheres with Raman tags flowing in a microfluidic system.
Using a typical micropipette or optical tweezers, a single MS-AuNet is placed on the spot of interest, trans-located to another spot and removed from the surface under the naked eye monitoring through an optical microscope. Since the SERS activity of a MS-AuNet is stable in water, this system can be utilized for in situ surface chemical probing in aqueous phase, which is useful for most of electro catalysis studies. The MS-AuNet in this work offers new ways for SERS-based probing techniques to a wide range of valuable applications such as a universal and reliable way of chemical identification with a high spatial resolution on surfaces, in vivo chemical or biological monitoring on the membrane of a living cell like neuron or stem cell, and high throughput decoding of microsphere suspension arrays in microfluidic systems.
Number | Date | Country | Kind |
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10-2009-0027800 | Mar 2009 | KR | national |