Quartz crystal microbalance and infrared reflection absorption spectroscopy characterization of bisphenol A absorption in the poly(acrylate) thin films

The absorption process of bisphenol A (BPA) in a number of poly(acrylate) thin films, such as poly(2-methoxyethyl acrylate) (PMEA), poly(ethyl acrylate) (PEA), poly(n-butyl methacrylate) (PBMA), and poly(methyl methacrylate) (PMMA), has been investigated by quartz crystal microbalance (QCM) and infrared reflection absorption spectroscopy (IRRAS) measurements. Both QCM and IRRAS measurements show that the BPA molecules absorb in PMEA, PEA, and PBMA thin films but not in PMMA thin film. The differences in the BPA absorption behavior are mainly attributed to the difference in the glass transition temperature (T(g)) between these polymers. This absorption behavior also depends on the BPA concentration and polymer film thickness. Furthermore, IRRAS characterization demonstrates that the hydrogen bonding is formed between the hydroxyl group in BPA and the carbonyl group in the poly(acrylate) thin films. BPA molecule absorbed in these polymer thin films can be removed by ethanol rinse treatment. By optimizing experimental conditions for the QCM electrode modified by PMEA thin film, detection limitation of approximately 1 ppb for BPA can be realized by the in situ QCM measurement. This method is expected to be a sensitive in situ detection way for trace BPA in the environmental study.     

Synthesis and characterization of a pH/temperature responsive glycine-mediated hydrogel for drug release

In this work, a pH/temperature responsive hydrogel (PMEA) from N-acryloylglycine methyl ester (NAGME), N-acryloylglycine ethyl ester (NAGEE), and acrylic acid (AAc) was synthesized by free radical polymerization. The swelling behaviors and drug release properties of hydrogels were systematically investigated at different temperature, pH, and AAc content. It was found that the hydrogel PMEA demonstrated pH and temperature responsive nature. The caffeine-release behaviors showed that only 49.1% caffeine was released from PMEA in pH 2.70 phosphate buffer solution (PBS) after 500 minutes, whereas more than 93.9% caffeine was gradually diffused into the medium in pH 7.49 PBS over the same time interval. In addition, the caffeine release was much higher at 37°C than that at 14°C in deionized water. As seen from the results, the PMEA seems to be a potential drug carrier with pH-temperature responsiveness.     

Synthesis and characterization of a pH/temperature responsive glycine-mediated hydrogel for drug release

In this work, a pH/temperature responsive hydrogel (PMEA) from N-acryloylglycine methyl ester (NAGME), N-acryloylglycine ethyl ester (NAGEE), and acrylic acid (AAc) was synthesized by free radical polymerization. The swelling behaviors and drug release properties of hydrogels were systematically investigated at different temperature, pH, and AAc content. It was found that the hydrogel PMEA demonstrated pH and temperature responsive nature. The caffeine-release behaviors showed that only 49.1% caffeine was released from PMEA in pH 2.70 phosphate buffer solution (PBS) after 500 minutes, whereas more than 93.9% caffeine was gradually diffused into the medium in pH 7.49 PBS over the same time interval. In addition, the caffeine release was much higher at 37°C than that at 14°C in deionized water. As seen from the results, the PMEA seems to be a potential drug carrier with pH-temperature responsiveness.     

Quantitative analysis of surface-immobilized protein by TOF-SIMS: effects of protein orientation and trehalose additive

We demonstrate the effects of protein orientation and trehalose on a quantitative analysis of surface-immobilized proteins by using time-of-flight secondary ion mass spectrometry (TOF-SIMS). As our model protein, streptavidin (SA) was quantitatively immobilized on a solid surface at different configurations by random or oriented immobilization and subsequently treated with trehalose. The resulting surface was analyzed by using TOF-SIMS and surface plasmon resonance (SPR) spectroscopy, where the secondary ion spectra from SA were compared with the surface density of the protein. In the case of oriented immobilization, the ion peak intensities measured by TOF-SIMS were correlated well with the SPR data, regardless of the presence of trehalose. Alternatively, trehalose significantly increased correlation between TOF-SIMS and SPR data for the randomly immobilized SA. It is likely that a trehalose-treated surface is less vulnerable to denaturation, thus leading to a reliable quantification of surface-immobilized proteins by TOF-SIMS. Our results show that TOF-SIMS can be used for understanding biophysical states such as orientation and denaturation of surface-immobilized proteins as well as for quantifying proteins within the field of biosensors and biochips.     

Plasmonic properties of polycrystalline hollow Au nanoparticles: a surface roughness effect

Hollow Au nanoparticles with a 25 nm polycrystalline shell and a 50 nm hollow core were produced in large amounts by using electrochemically evolved hydrogen nanobubbles as templates and reducing agents for electroless deposition from a Na3Au(SO3)2 electrolyte. The surface roughness of these nanoparticles can be tuned by adding NaSO3 into the electrolyte. Different surface roughnesses can be readily obtained for sub-100 nm particles with the same size. As surface roughness increases, surface plasmon resonance (SPR) peaks shift to longer wavelengths. Particles with an 8 nm roughness have a SPR peak centered at 750 nm, which is particularly attractive for in vivo diagnostic and therapeutic applications. A three-dimensional finite difference time domain (FDTD) simulation confirms that the red-shifts of SPR peaks are mainly caused by their surface roughness, and the hollow nature of these particles plays only a minor role. This unique plasmonic property of hollow Au nanoparticles opens up the possibility to maintain the desirable optical properties after loading other substances into the hollow core to form multifunctional core-shell nanoparticles.     

Surface-modified mesoporous silicas as recyclable adsorbents of an endocrine disrupter, bisphenol A

Surface-modified mesoporous silicas (MSs) were investigated for recyclable adsorption of an endocrine disrupter, bisphenol A (BPA). Surface-modified MSs were prepared by (i) post-synthesis surface modification of MSs using surface hydroxyl groups and organosilanes (m-MS) and by (ii) co-condensation of tetraethoxysilane and the corresponding organosilanes (d-MS). Infrared measurements indicated that organic groups mainly existed on the surface of m-MS, which resulted in a surface characterized by high hydrophobicity. Both organic groups and isolated hydroxyl groups existed on the surface of d-MS, resulting in both hydrophobicity and hydrophilicity on the surface. The amount of BPA adsorbed on surface-modified MSs per organic group was similar for m-MS and d-MS, however, the d-MS established equilibrium for BPA adsorption faster than m-MS, as measured by UV-vis spectra. A larger amount of BPA per surface area could be adsorbed on carbon materials than on the surface-modified MSs, however, the regeneration of carbon materials by washing could not be done easily. The surface-modified MSs retain adsorption capacity for BPA after several regeneration cycles, demonstrating that the surface-modified MSs are effective recyclable adsorbents of the endocrine disrupter, bisphenol A.     

Fabrication of a surface plasmon resonance biosensor based on gold nanoparticles chemisorbed onto a 1,10-decanedithiol self-assembled monolayer

We have investigated the fabrication of surface plasmon resonance (SPR) biosensors using self-assembled monolayers (SAMs) and adsorbed gold nanoparticles. The SAM of 1,10-decanedithiol was first fabricated onto a gold substrate. Gold nanoparticles were then chemisorbed onto the SAM surface by bonding with the terminal thiol groups, forming a sensor that can be used to immobilize proteins. Bovine serum albumin (BSA) was used as a test protein in this study. Several spectroscopic and microscopic techniques were used to investigate both the SAM and the chemisorption of gold nanoparticles at the SAM surface. Our results confirm the covalent bonding of the gold nanoparticles onto the SAM. Surface plasmon resonance (SPR) was used to study both the adsorption of BSA to the SAM surface and to the gold nanoparticle-coated SAM. For SAM surfaces with adsorbed gold nanoparticles a larger SPR response to BSA than to the sensors with a bare SAM is observed.     

Electrochemical thinning of thicker gold film with qualified thickness for surface plasmon resonance sensing

To meet the requirement of surface plasmon resonance (SPR) sensing, controlling the thickness of the gold film is very important. Here, we report an efficient and simple approach to prepare a SPR-active substrate when the thickness of the gold film is larger than the optimizing 50 nm and smaller than 100 nm. This method is based on anodic electrodissolution of gold in electrolyte containing chloride ions. Using this method, the thickness of gold films can be easily changed at a nanometer scale by controlling the number of potential scans and the concentrations of chloride ions in the electrolyte. At the same time, the influence of gold film thickness on the SPR signal is recorded by SPR in real time. To assess the change of the surface roughness and morphology of gold film through anodic electrodissolution, atomic force microscopy was used. The surface roughness of the same Au film before and after anodic electrodissolution is 1.179 and 2.767 nm, respectively. The change of the surface roughness of Au film brings out a slight angle shift of SPR. This indicates that surface electrodissolution of the gold does not affect the character of the original bulk film and this film can be used for SPR experiments. To confirm our expectation, a simple adsorption experiment of cytochrome c (Cyt c) on the gold film treated with anodic electrodissolution modified by 11-mercaptoundecanic acid was carried out. The angle shift of SPR confirmed the adsorption of Cyt c, and the cyclic voltammetry of Cyt c provided a complementary confirmation for the adsorption of Cyt c. These results show that this approach provides a good way to change the thicker gold film to an optimized thickness of SPR sensing. The great advantage brought by this approach is in that it can convert the waste gold films with greater thicknesses fabricated by the vacuum deposition method or other methods into useful materials as active SPR substrates.     

Effect of varying electric potential on surface-plasmon resonance sensing

The high sensitivity of surface-plasmon resonance (SPR) sensors allows measurements of small variations in surface potentials to be made. We studied the changes of the SPR angle when an oscillating electric potential was applied to a gold film on which surface plasmons were excited. The shifts of the SPR resonance angle were observed for various aqueous solutions as an adjacent medium. A model that takes into account the redistribution of charges at the double layer near the metal-liquid interface as well as the oxidation of the gold film was developed. It was found that a change in the electronic density at voltages below the oxidation potential and, in addition, the oxidation of the gold surface above this potential are the main mechanisms that account for the observed dependences. It was shown that relatively slow oxidation-reduction processes can explain the observed hysteresis effect. Application of these techniques to studies of dielectric properties and conformational changes of polar biomolecules, such as tubulin, are discussed.     

Improved sensitivity of wavelength-modulated surface plasmon resonance biosensor using gold nanorods

In this report, gold nanorods (GNRs) were used to enhance the sensitivity of the wavelength-modulated surface plasmon resonance (SPR) biosensor. The GNRs were designed and fabricated through seed-medicated growth and surface activation by a layer of a weak polyelectrolyte, poly(acrylic acid) for the attaching antibody. Rabbit anti-goat IgG was immobilized on GNRs, and sandwich assays were carried out to detect goat IgG using a wavelength-modulated SPR biosensor. The detection sensitivity of the nanorod-conjugated antibody is 25-100 times more sensitive than the SPR biosensor without GNRs. Drastic sensitivity enhancement, owing to the electromagnetic interaction between the nanotag and the sensing film, was maximized using the longitudinal plasmonic resonance of the GNRs. GNRs could significantly enhance the sensitivity of the SPR biosensor, and the maximum enhancement effect can be achieved when the longitudinal SPR peak wavelength of GNRs functionally matches the surface plasmon wavelength.     

Side-polished multimode fiber biosensor based on surface plasmon resonance with halogen light

A side-polished multimode fiber sensor based on surface plasmon resonance (SPR) as the transducing element with a halogen light source is proposed. The SPR fiber sensor is side polished until half the core is closed and coated with a 37 nm gold thin film by dc sputtering. The SPR curve on the optical spectrum is described by an optical spectrum analyzer and can sense a range of widths in wavelengths of SPR effects. The measurement system using the halogen light source is constructed for several real-time detections that are carried out for the measurement of the index liquid detections for the sensitivity analysis. The sensing fiber is demonstrated with a series of refractive index (RI) liquids and set for several experiments, including the stability, repeatability, and resolution calibration. The results for the halogen light source with the resolution of the measurement based on wavelength interrogation were 3 x 10(-6) refractive index units (RIUs). The SPR dip shifted in wavelength is used as a measure of the RI change at a surface, and this RI change varies directly with the number of biomolecules at the surface. The SPR dip shift in wavelength, which was hybridized at 0.1 microM of the target DNA to the probe DNA, was 8.66 nm. The all-fiber multimode SPR sensor, which has the advantages of being low cost, being disposable, having high stability and linearity, being free of labeling, and having potential for real-time detection, permit the sensor and system to be used in biochemical sensing and environmental monitoring.     

Wavelength-scanning surface plasmon resonance imaging

A new surface plasmon resonance (SPR) imaging technique was proposed. After measurements were conducted at varying wavelengths, the wavelength affording the minimum brightness (SPR wavelength) was determined at each pixel of the image. A two-dimensional map of the SPR wavelength could be converted to a thickness profile by use of a nonlinear calibration curve, which was obtained by Fresnel calculation. An array of protein thin layers on a gold film was evaluated in air to present the layers' surface structure in nanometer scale.     

Label-free detection of proteins in crude cell lysate with antibody arrays by a surface plasmon resonance imaging technique

We established a label-free method of measuring proteins in crude cell lysate using antibody arrays and surface plasmon resonance (SPR) imaging. The refractivity of the running buffer was adjusted with that of the lysate to overcome the bulk effect. The chemistries of the fabricated arrays were investigated to reduce nonspecific adsorption on the array surface. We found that the hydrophilicity of the poly(ethylene glycol) moiety and lower electrostatic charge on the surface provided a specific measurement of antigen-antibody interaction. We validated the system by measuring the expression of eight proteins in the mouse brain and comparing the results to those by conventional Western blotting. The detection limit of the antibody array was approximately 30 ng/mL in crude cell lysate, on the same order as that of previous SPR research. The system enabled quick, label-free, and high-throughput analysis of abundant proteins with minimal sample volume ( approximately 200 muL). It is expected that our SPR antibody array will be applicable for direct protein expression profiling of cell lysate, as well as for cell phenotyping, food analysis, discovery of new biomarkers, and immunological disease diagnostics.     

GSH、L-Cys和6-MP诱导纳米金聚集行为的分析与应用

利用纳米金的表面等离子共振吸收光谱研究了谷胱甘肽(GSH)、L半胱氨酸(L-Cys)和6-巯基嘌呤(6-MP)3种巯基化合物与纳米金作用的机理.结果表明,3种巯基化合物均能诱导纳米金聚集导致其表面等离子共振吸收峰红移.聚集后的纳米金对介质环境(如溶液极性)的变化较敏感,其吸收峰随着加入分析物的极性变化有规律地红移,并且吸光度也线性下降.可根据波长的红移或吸光度变化确立定量关系,可望用于分析对象的定量检测.     

ssDNA aptamer-based surface plasmon resonance biosensor for the detection of retinol binding protein 4 for the early diagnosis of type 2 diabetes

Retinol binding protein 4 (RBP4) is a useful biomarker in the diagnosis of type 2 diabetes since its level in the serum is higher in insulin-resistant states. Accurate measurement of the serum RBP4 levels is hampered by conventional immunologic methods, such as enzyme-linked immunosorbent assay (ELISA). In this study, therefore, we have developed an aptamer-based surface plasmon resonance (SPR) biosensor that can be used to sense for RBP4 in serum samples. A single-stranded DNA (ssDNA) aptamer that showed high affinity (Kd = 0.2 +/- 0.03 microM) and specificity to RBP4 was selected. This RBP4-specific aptamer was immobilized on a gold chip and used in a label-free RBP4 detection using SPR. Analysis of RBP4 in artificial serum using SPR was compared with ELISA and Western blot analysis. Our results indicated that the RBP4-specific aptamer-based SPR biosensor gave better dose-dependent responses and was more sensitive than ELISA assays. As such, this RBP4 aptamer-based SPR biosensor can be potentially used to monitor the RBP4 levels within the serum as an indicator of type 2 diabetes.     

Enzymatically amplified surface plasmon resonance imaging detection of DNA by exonuclease III digestion of DNA microarrays

This paper describes a novel approach utilizing the enzyme exonuclease III in conjunction with 3'-terminated DNA microarrays for the amplified detection of single-stranded DNA (ssDNA) with surface plasmon resonance (SPR) imaging. When ExoIII and target DNA are simultaneously introduced to a 3'-terminated ssDNA microarray, hybridization adsorption of the target ssDNA leads to the direction-dependent ExoIII hydrolysis of probe ssDNA strands and the release of the intact target ssDNA back into the solution. Readsorption of the target ssDNA to another probe creates a repeated hydrolysis process that results over time in a significant negative change in SPR imaging signal. Experiments are presented that demonstrate the direction-dependent surface enzyme reaction of ExoIII with double-stranded DNA as well as this new enzymatically amplified SPR imaging process with a 16-mer target ssDNA detection limit of 10-100 pM. This is a 10(2)-10(3) improvement on previously reported measurements of SPR imaging detection of ssDNA based solely on hybridization adsorption without enzymatic amplification.     

离子交换波导折射率分布峰值的移动

本文提出利用表面等离子波共振(SPR)技术测量离子交换波导的表面折射率的方法.这种方法基于分析转移矩阵方法和表面等离子波技术,避免了传统方法中确定表面折射率的自由度问题,所以得到的波导表面折射率要比逆WKB法更为精确.用此方法研究了不同交换时间的样品,我们发现当交换达到一定的时间后,表面折射率会随交换时间的增长而下降,折射率分布峰值移向波导内部,说明这时存在一个银离子反扩散的过程,这与理论分析结论相吻合.     

Postcolumn renewal of sensor surfaces for high-performance liquid chromatography-surface plasmon resonance detection

The combination of high-performance liquid chromatography (HPLC) with surface plasmon resonance (SPR) for continuous separation and label-free detection of protein samples is described. The detection was realized by electrostatic adsorption of proteins bearing positive and negative charges onto chemically modified SPR sensors in two separate SPR channels. One SPR channel is coated with carboxymethylated dextran which facilitates the detection of positively charged proteins, whereas the other, devoted to the monitoring of negatively charged proteins, is covered with ethylenediamine molecules attached onto a dextran surface. Renewal of the sensor surface in the channels can be accomplished by introducing regeneration solutions through two six-port valves. The coupled technique (HPLC-SPR) was assessed for its analytical figures of merit and applied to the quantification of lysozyme in human milk samples. Unlike the SPR detection of bulk solution refractive index changes during chromatographic peak elutions, the highest sensitivity of SPR is retained in this work since the measurement is performed at the SPR sensor surface where the evanescent field is the strongest. The renewable SPR detection of continuous separations is reproducible and versatile and does not require the separated proteins to contain chromophores or to be prelabeled with a tag (e.g., a redox-active or fluorescent molecule). Such generality makes SPR complementary to other types of chromatographic detectors.     

Detection of heavy metal ions in water by high-resolution surface plasmon resonance spectroscopy combined with anodic stripping voltammetry

High-resolution differential surface plasmon resonance (SPR) with anodic stripping voltammetry (ASV) capability has been demonstrated for detecting heavy metal ions in water. Metal ions are electroplated onto the gold SPR sensing surface and are quantitatively detected by stripping voltammetry. Both the SPR angular shift and electrochemical current signal are recorded to identify the type and amount of the metal ions in water. The performance of the combined approach is further enhanced by a differential detection approach. The gold sensor surface is divided into a reference and a sensing area, and the difference in the SPR angles from the two areas is detected with a quadrant cell photodetector as a differential signal. Our system demonstrated quantitative detection of copper, lead, and mercury ions in water from part-per-million to sub-part-per-billion levels with good linearity.