shell Nanoparticles

CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 37, Issue 12, December 2009 Online English edition of the Chinese language journal

Cite this article as: Chin J Anal Chem, 2009, 37(12), 1847–1852.

REVIEW

Study Progress on Biosensing Core/shell Nanoparticles LUO Jie, ZENG Guang-Ming*, TANG Lin, YIN Juan, LI Yuan-Ping College of Environmental Science and Engineering, Hunan University, Changsha 410082, China

Abstract: Recently, considerable attention has been paid to nanomaterials in various fields. Especially, the preparation methods of core/shell nanoparticles have been drastically updated and developed. There still exists a great application prospect for the development of biosensing core/shell nanoparticles. This article emphatically introduces the operation principle, preparation methods of biosensing core/shell nanoparticles, and the recent application progress in electrochemical biosensors, optical biosensors, and piezoelectric crystal biosensors. Key Words: Core/shell; Nanoparticle; Preparation; Biosensor; Review

1

Introduction

Nanotechnology is a novel technique developed in the 1980s that has been widely applied to various fields, such as materials, optics, chemical industry, medicine, and environment protection[1]. Nanometer materials have become a hot subject of research in material science. Nanoparticles are atomic or molecular clusters composed of an extremely small amount of atoms or molecules, forming a typical meso system between the macroscopical matter and the microcosmic atom or molecule, and their sizes are between 1 and 100 nm. Their special structures provide many unique characteristics, such as surface effect, small size effect, quantum size effect, macroscopic quantum tunnel effect, and so on. People almost always pursue subminiature, highly sensitive, and fast responsive biosensors[2], meanwhile, nanoparticles satisfy these demands. Core/shell nanoparticles can ulteriorly improve the performance of nanometer materials, besides, the combination of various composition approaches and the unique characteristics of nanoparticles make the composition technology be another highlight in material field. Core/shell nanoparticles can be made up of many kinds of materials, including macromolecules, minerals, metals, and so on. The shell coated outside can change the

optic, electric, or magnetic properties of the nanoparticles, so the nanoparticles have a great prospect of application in biosensors.

2

Working principle of biosensing core/shell nanoparticles

Nanoparticles exhibit their superiority during application in biosensors for their excellent characteristics, such as large specific surface area, good biocompatibility, and electrocatalysis. Nanoparticles can be applied in biosensors via constructing active interface to immobilize biomaterials or acting as labels. 2.1

Construction of active interface to immobilize biomaterials

A nanoparticle constructs an active interface to immobilize biomaterials since it can keep the activity of biological components and speed up the electron transfer. The process of a nanoparticle fixing antibody that combines with specific antigen to form a core/shell nanoparticle is shown in Fig.1. Biomolecules, such as deoxyribonucleic acid (DNA) and enzyme, are easily inactivated during the process of detection,

Received 21 April 2009; accepted 20 June 2009 * Corresponding author. E-mail: [email protected], [email protected] This work was supported by the National Natural Science Foundation of China (No. 50608029), the Hunan Provincial Innovation Foundation For Postgraduate, the Chinese National Basic Research Program (973 Program; No. 2005CB724203), the Program for Changjiang Scholars and Innovative Research Team in University of China (No.IRT0719). Copyright © 2009, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(08)60152-8

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Fig.1 Nanoparticle constructing active interface

but if the biomolecule is fixed on the nanoparticle, it can not only retain the activity but also the response signal of the biomolecule will be stronger than that of a single material used, because the nanoparticles combine the advantages of different materials and have a good biocompatibility. Feng[3] first developed CeO2/chitosan composite matrix for the immobilization of single-stranded DNA (ssDNA) probe and the fabrication of DNA biosensors to detect colorectal cancer gene. The nanoparticles enhanced the loading of ssDNA probe on the surface of electrode with nontoxicity and excellent electronic conductivity. Peptide nucleic acid oligomers could also be fixed on the nanoparticles and specifically hybridized with complementary DNA molecules[4]. In Tang’s research[5], an electrochemical biosensor based on the immobilization of laccase on magnetic core/shell (Fe3O4/SiO2) nanoparticles was constructed for the determination of catechol concentration in compost. The immobilization matrix provided a good microenvironment for retaining the laccase bioactivity. Moreover, carbon nanotubes and chitosan have been used to fabricate a stable ultra-thin multilayer film on a gold electrode in a layer-by-layer fashion, and according to this, Yang designed a kind of biosensors to detect cholesterol[6]. Nanoparticles can greatly improve the catalytic activity of the enzyme and increase the current response sensitivity, and furthermore, it can also improve the capability of antidisturbance. The key technology for fabricating biosensors with core/shell nanoparticles is to fix up the biomaterial (enzyme, antigen, antibody, biological tissue, cell, and DNA) on the surface of biosensor’s transducer (basal electrode, crystal oscillator, and photoelectrode) stably, retaining the high activity and optimizing the biosensor’s characteristics, such as repeatability, sensitivity, linear range, detection limit, and service life. The main methods for immobilizing the biomaterial on the surface of the transducer include adsorption, embedding, crosslinking, covalent bonding, and directional fixation methods. 2.2

Application of biosensing core/shell nanoparticles as labels in biosensors

The main test methods of core/shell nanoparticles are spectrophotometry, fluorimetry, and electrochemical methods. Spectrophotometry is used according to the change of principal absorption wavelength before and after the

immobilization of DNA probe labeled by nanoparticles. The excitation spectrum of the nanoparticles is wide and continuous, but emission spectrum is symmetry and narrow, and otherwise, the color of emission spectrum is adjustable. That is to say, nanoparticles with different sizes can be excitated by single wavelength of light and they emit different color of stable light, so fluorimetry can be used to detect the label. Electrochemical method is a novel technique to detect the nanoparticle labels. Most nanoparticles used for labels are composed of metal atoms or semiconductor materials. The oxidation-reduction potential is different for different metals, so nanoparticle label can be determined by analyzing the metal content. The common electrochemical methods are cyclic voltammetry, stripping voltammetry, differential pulse voltammetry, and so on. An electrochemical detection method for analyzing sequence-specific DNA via a gold nanoparticle DNA probe and a subsequent signal amplification step by silver enhancement was described by Cai et al[7]. The sensitivity of the electrochemical biosensors was increased by approximately two orders of magnitude, and a detection limit of 50 pM of complementary oligonucleotides was obtained by using differential pulse voltammetry to detect silver. Magnetic beads were introduced in the detection of DNA sequence by Palecek[8]. Oligonucleotide was labeled by magnetic beads and hybridized with target DNA, then separated with the help of magnetic force. They detected DNA by using cathodic stripping voltammetry, and the detection limit was lowered. Duan[9] first prepared the fluorescent nanoparticles consisting of a core of fluorescein isothiocyanate labeled with goat anti-human immunoglobulin (IgG). This technique overcomes the problem of the leakage of fluorescent dyes in traditional method. The core/shell fluorescent nanoparticles are much smaller than cells and biocompatible, and they provide novel materials for biosensors, and besides, the labeling method based on core/shell fluorescent nanoparticles has provided a novel nonisotopic analytic technique for biomedicines. Cai[10] reported a low-temperature method for generating core-shell particles consisting of a Cu core and a thin layer of Au shell that could be readily functionalized with oligonucleotide. Core/shell Cu/Au particles were successfully labeled to a 5’-alkanethiol capped oligonucleotide probe. The target oligonucleotides were electrostatically adsorbed onto the surface of a glassy carbon electrode and then hybridized to the alloy particle-oligonucleotides DNA probe. The electrical signal was enhanced more obviously in the process of hybridization by Cu/Au than by single Au nanoparticles. Huang[11] successfully detected Į-human-IgE-biotin complex using CdSe/ZnS core shell nanoparticles and then used quantum dots as indicator to make quantitative analysis of urea[12]. The particular characteristics of core/shell

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nanoparticles make biosensors have strong adaptability and high sensitivity. Hun[13] proposed a highly sensitive sandwich fluoroimmuno assay based on the functionalized fluorescent core/shell nanoparticles as the label coated with antiStaphylococcal enterotoxin C1 monoclonal antibody. The calibration graph for Staphylococcal enterotoxin C1 was linear over a range of 1.0–75.0 ȝg l–1 with a detection limit of 0.3 ȝg l–1. Core/shell nanoparticles show their special optical properties, and catalytic property differed with traditional materials and ameliorate the capability of biosensors, such as response sensitivity and anti-interference ability.

3

Preparation of biosensing core/shell nanoparticles

The main preparation methods of biosensing core/shell nanoparticles are sol-gel method, deposition method, in-situ polymerization, self-assembly method, intercalation method, chemical coprecipitation, emulsion polymerization, and so on. The following sections emphasize four commonly used preparation methods. 3.1

Sol-gel method

Sol-gel method is an important technique of nanoparticle composition. In the system of polymer cosolvent, the precursors, such as alkoxyl oxygen metals or metal salts, are hydrolyzed or condensed, and the nanoparticles are prepared if the condition is controlled properly and the polymer is not separated in the process of gel formation and drying. The main advantage of this technology is its low costs, simple craftwork, and improved component control. Sol-gel method plays an important role in the preparation and structure controlling of nanocomposite materials, such as semiconductor/SiO2 nanoparticles. The method needs only a simple equipment and a low reaction temperature, and it can be used to prepare compounds which are difficult to prepare by other methods. A simple sol-gel-based core/shell approach for the synthesis of alumina-aluminium titanate composite was reported by Jayasankar[14]. This method is effective in controlling the grain size of nanoparticles, the low temperature formation, and sintering of the composite at a lower temperature. The low temperature formation of aluminum titanate in the alumina matrix could be attributed to a maximum contact area between the reactants in the core-shell approach. The present approach results in good densification without significant grain growth. 3.2

Deposition method

Deposition method is to scatter a metal rapidly to the preconcerted surface, and then a polymer is heated and

evaporated immediately to the underlay, or a common monomer or polymer produced monomer is scattered to the underlay by high temperature pyrolysis, which can be aggregated with metal scattering at the same time or polymerized after aggregation. Au-TiO2 nanoparticles were prepared via depositionprecipitation method by Ashokkumar[15]. When TiO2 nanotubes array was modified by CdS, the process of CdS grain depositing to the underlay was actually a process of surface adsorption-nucleation. Adsorption ability of the membrane was weak, and the density of nucleation was low when the surface was slick. On the contrary, the underlay had high activity and adsorption ability, when the surface was relevantly coarse[16]. Flores[17] prepared SiO2@Ag core-shell nanospheres, which were silver nanoparticles ((4 ± 2) nm in diameter) coated silica nanospheres ((50 ± 10) nm in diameter). The nanoparticles were prepared, with tetraethylorthosilicate as Si source and silver nitrate as Ag source in a water/ethanol via a single-pot wet chemical route without adding coupling agent or surface modification, which led to the formation of core/shell homogeneous nanospheres. In the process of preparation, silver nanoparticles were deposited from Ag+ in the solution. Repeated depositions of polyaniline were used to control the thickness of the polymeric film deposited on poly(vinyl chloride) membrane surface. The oxidation of aniline was carried out in a dispersion mode in the presence of poly(N-vinylpyrrolidone)[18]. 3.3

In-situ polymerization

Molecular composites are formed when in-situ polymerization occurs after a rigid polymer is dissolved in flexible polymer monomer, and the generated rigid polymer is dispersed homogeneously in flexible polymer style. The technology of in-situ polymerization improves the dispersity of the nanoparticles in polymer style to a great extent, and it also improves the mechanical property of composite materials. The content of crosslinking agent has a great influence on the size of nanoparticles in the process of preparation. Ren[19] prepared thermo-sensitive polylactic acid/isopropylacrylamideco-acrylamide core/shell micelle with different response temperatures. Fluid mechanics diameter of the nanoparticles increased from 886.5 to 170.2 nm at 25 ºC when the mole fraction of the crosslinking agent increased from 5% to 15%. Liu[20] synthesized the SiO2/polypyrrole particles and the thickness of shell was controllable. Ag/polyaniline core/shell nanocomposites were synthesized via in-situ chemical oxidation polymerization by Jing[21]. A method was developed for the preparation of core/shell attapulgite@polyaniline (ATP@PANI) composite particles via in-situ oxidative polymerization. The effects of the concentration of

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hydrochloric acid on the morphology and the electrical conductivity of ATP@PANI composite particles were also studied[22]. Luo et al[23] illustrated the interaction of an electropolymerized polyaniline film with silica microparticles. Silica particles coated with a nanofilm of polyaniline appear to adsorb horseradish peroxidase more effectively, meanwhile also to increase the active electrode surface area. 3.4

Self-assembly method

Self-assembly method is the process that the module units spontaneously form thermodynamically stable congeries with a definite ordered structure. The driving force of the shell of the nanoparticles prepared via self-assembly method is electrostatic attraction brought by the opposite charges of particles among the center and lamella or adjacent lamellae. It is a process of physical adsorption. It can also improve the stability of adjacent lamellae by triggering chemical reaction. Sodium lauryl sulfate monofilm and titanium dioxide nanoparticles’ film were alternately assembled on the surface of SiO2 microballoon spheres[24]. TiO2/SiO2 core/shell nanoparticles were obtained by calcining the complex film at a high temperature. The nanoparticles were characterized by x-ray diffraction, scanning electron microscopy, x-ray energy spectrum, and so on. The research showed that TiO2 was arranged compactly and homogeneously on the surface of compound nanoparticles. Au/C[25], CdS/polyelectrolyte[26] core/shell nanoparticles were synthesized via self-assembly method. Bizdoaca[27] prepared core/shell nanoparticles using the self-assembly method. The core-shell particles consisted of a polystyrene spherical core with a diameter of 640 nm covered with a shell of five layers of Fe3O4 nanocrystals with a diameter of 12 nm. A novel class of biofunctional fluorescent microparticles for application in immunoassays was constructed via the layer-by-layer self-assembly method to deposit multiple layers of fluorescein-labeled polyelectrolytes onto the colloidal particles, followed by the deposition of a protein layer, and the experimental results show a good effect[28]. Micelle-supported gold composites with a polystyrene core and a poly(4-vinyl pyridine)/Au shell were synthesized by Chen[29]. Research results indicate that composites with high content of gold showed higher catalytic activity and higher catalytic efficiency. Wang[30] prepared Au-C@SiO2 complex and fabricated hydrogen peroxide biosensors.

4

Application of core/shell nanoparticles in biosensors

Core/shell nanoparticles are applied extensively in biosensors, such as electrochemical biosensors, optical biosensors, and piezoelectric crystal biosensors because of their excellent characteristics.

4.1

Application of core/shell nanoparticles in electrochemical biosensors

Electrochemical biosensor is equipment made up of sensitive membrane and transducer, such as electrochemical electrodes, which transfers biomass to electric signal. The sensitive membrane can be made up of enzyme, microorganism, antigen, antibody, cell, or animal vegetable tissue. People have great interest in applying various core/shell nanoparticles in the research of electrochemical biosensors and make great progress in the sensor technology. Figure 2 is a principle graph showing that the electrode is modified by the chemical group, and core/shell nanoparticles which are formed by adsorption and deposition are fixed on the electrode through the chemical group, finally, the biomolecules are connected on the surface of nanoparticles, and then the nanoparticles can be used in detection. Cai et al[7] reported that the sensitivity of the electrochemical DNA biosensors labeled by Au/Ag nanoparticles is increased by approximately two orders of magnitude than that labeled by single nanoparticles. The phenol biosensors were developed based on the immobilization of tyrosinase on the surface of the modified magnetic MgFe2O4 nanoparticles. The magnetic bionanoparticles were attached to the surface of carbon paste electrode (CPE) with the help of a permanent magnet, then phenol was detected via electrochemical techniques, such as cyclic voltammetry[31]. Si/Au core/shell nanoparticles were self-assembled on the surface of 3-aminopropyltrimethoxysilane-(APTES) modified indium tin oxide (ITO) electrode. The resulting gold nanoshells (GNSs)-coated ITO (GNSs/APTES/ITO) electrode could provide a large biocompatible surface and electrical conductivity, and it could act as electron tunnels to facilitate electron transfer between the hemoglobin and the electrode. Moreover, Wang constructed a hydrogen peroxide biosensor according to this mechanism[32]. Zhang et al[33] detected hydroquinone with the CPE modified with Fe3O4/SiO2 nanoparticles. Qiu et al[34] developed a novel amperometric glucose biosensors using Fe3O4/SiO2 nanoparticles modified with ferrocene. The obtained magnetic bio-nanoparticles attached to the surface of a CPE and acted as a mediator to transfer electrons between the enzyme and the electrode. This biosensor is able to detect glucose in linear range from 1.0 × 10–5 M to 4.0 × 10–3 M. Silver chloride@polyaniline core/shell nanocomposites were synthesized and used in constructing biosensors by Yan[35]. The nanocomposites showed excellent electrochemical behavior in pH neutral environment and had inhibitive effect on the oxidation of ascorbic acid. The biosensors could detect dopamine at its very low concentration in the presence of ascorbic acid with 5000 times concentration of it.

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Fig.2 Principle flow chart of core/shell nanoparticles applied in electrochemical biosensors

4.2

Application of core/shell nanoparticles in optical biosensors

Optical biosensors can recognize the information of molecules selectively, induce optical change, and then transfer the optical change into electric signal. There are many optical signals, such as light absorption, fluorescence, surface plasmon resonance, and so on. Optical biosensor has a high sensitivity and does not need reference, and the light propagation signal is not easy to be interfered by electromagnetism outside. Fig.3 shows an illustrative diagram of core/shell nanoparticles applied in optical biosensors. First, a bottom layer is assembled on the surface of plate, then nanoparticles which act as core are connected on the bottom layer and covered by shell layer, then the particles can be used in detection due to their specific optical characteristics. Endo[36] proposed a novel label-free cell-based assay that was based on a localized surface plasmon resonance (LSPR) biosensors, which is excited using core-shell structured nanoparticle layer substrate. The refractive index change caused by the specific interaction between the antigen and the antibody was detected on the antibody immobilized LSPR-based biosensors. The optical characteristics were monitored for the detection of specific reactions between antibody and cell metabolites, and the detection limit of the biosensors was 10 pg ml–1. A type of worm-like nanorods was synthesized by Huang[37] using conventional gold nanorods reacting with Na2S2O3 or Na2S. The formed worm-like nanorods possessed higher sensitive property in LSPR than the original nanorods. These features make the gold nanorods to have potential application in biomolecular recognition study and biosensor fabrications. Nanoparticles can be used in light absorption because of their specific optical characteristics. The specific antibody-antigen reaction was detected by Enders[38] by taking surface-enhanced infrared absorption (SEIRA). Single Au nanoparticles were deposited on the SiO2/Si wafer surface for forming SEIRA active film. After immobilizing the specific antibodies onto the AuNP, these samples were then exposed to specific antigens, and then the samples were studied with infrared spectroscopy.

4.3

Application of core/shell nanoparticles in piezoelectric crystal biosensors

Piezoelectric crystal biosensors is a novel biological detection method, which combines high-sensitivity piezoelectric sensors technology with differential biological response, and it transfers biological signal into physical or chemical signal, which is easy to detect qualitatively or quantitatively. Piezoelectric crystal has high-sensitivity quality response, and the frequency change is related to quality of material attached to the biosensors. Obviously, component detection with biological affinity can be used for fabricating the piezoelectric biosensors. The key step for designing biosensors is fixing immune active materials up to the surface of biosensors. A new material formed by assembling gold nanoparticles onto nano-sized hydroxyapatite was used for the interface design of piezoelectric immunosensors. The detection performance of the resulted immunosensors was studied via the antibody-antigen model system of Į-fetoprotein. Ding[39] also compared the immunoresponse of the proposed immunosensors with those of antibodies immobilized on hydroxyapatite or gold nanoparticles alone. The piezoelectric quartz crystal microbalance was used by Jia[40] to monitor the adhesion of cells on a chitosan/multiwalled carbon nanotube composite-modified gold electrode. The morphology and chemical properties of the film were characterized with scanning electron microscopy and infrared spectroscopy. Research shows that the nanocomplex has better biocompatibility with cells.

Fig.3

Principle flow chart of core/shell nanoparticles applied in optical biosensors

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5

Prospect

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Core/shell nanoparticles have excellent characteristics due to their special structures, and they have both the properties of nuclear layer and shell layer, so they have new optical or electrochemical characteristics which are different from those of single material. The previous research mainly focused on the field of core/shell polymer particles, but now the research has been deepened to the core or shell composed of polymers, oxides, noble metals, and so on, and the composition, morphology, and surface characteristics of core/shell nanoparticles are all tunable. The nanoparticles have important prospect of application. Biosensors have come into general application to all kinds of fields, such as immunology, medicine, and foodstuff, and they are also applied in the analysis and detection of trace toxicants in the environment[41]. Biosensing core/shell nanoparticles will attract considerable attention as novel compound materials. Quantum dot can send different colors of light when the sizes or composing materials are different or even a single wavelength of light is used to excite them because of quantum size effect, so it adapts more to the analysis of biomacromolecule nowadays and is extensively applied to biological markers. People also pay considerable attention to the research of biodegradable nanoparticles which encapsulate active materials. The nanoparticles make the active materials to separate from the surrounding media to prevent them from degrading too fast and release them when they are needed. The application of core/shell nanoparticles in biosensors improves the performance of biosensors and makes the detection limit of target lower, which makes biosensors research much deeper. But, some formation mechanism of core/shell materials and synergistic effect mechanism of cores and shells are still unclear. Some preparation technology is not consummate enough. The characteristics of acquired nanocomplex do not meet people’s expectations completely. Further study is needed to broaden the application fields of core/shell nanoparticles.

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