Longhua Guo

Oriented Gold Nanoparticle Aggregation for Colorimetric Sensors with Surprisingly High Analytical Figures of Merit

The common drawbacks of current colorimetric sensors using gold nanoparticle aggregation is its relatively low sensitivity and narrow dynamic range, which restrict their application in real sample analysis when competing with other analytical techniques such as fluorescence and chemiluminescence. In this article, we demonstrate a novel strategy to construct colorimetric sensors based on gold nanoparticle aggregation. Unlike the conventional colorimetric sensors which cause the formation of large nanoparticle aggregates, in our strategy, dimers are selectively formed upon target binding, which results in significantly improved long-term stability and a more than 2 orders of magnitude wider dynamic range of detection than that of the conventional colorimetric sensors. In addition, a strategy to minimize the interparticle gap through the formation of a Y-shaped DNA duplex enables to increase the limit of detection by 10,000 times. The analytical figures of merit of the proposed sensor are comparable to those of the fluorescence-based sensors.

Metal–organic framework (MOF): a novel sensing platform for biomolecules

The metal-organic framework (MOF) was first utilized as the sensing platform for assaying biomolecules. It has also been demonstrated that this novel strategy is effective and reliable for detection of HIV DNA and thrombin with high sensitivity and selectivity.

Three-Dimensionally Assembled Gold Nanostructures for Plasmonic Biosensors

Three-dimensional gold nanoarchitecture was fabricated by layer-by-layer (LbL) deposition of gold nanoparticles (AuNPs) and multiwalled carbon nanotubes (MWCNTs) on a glass substrate for a highly sensitive plasmonic biosensor using a conventional UV-vis instrument. Carboxyl-functionalized MWCNTs were reacted with 3-mercaptopropyltriethoxysilane (MPTES) to introduce multiple thiol groups onto MWCNTs. A self-assembled monolayer (SAM) of AuNPs on a glass chip was sequentially dipped into MPTES-functionalized MWCNTs (MWCNT-Si-SH) and AuNPs to form multilayers of AuNPs on MWCNTs. Such three-dimensionally assembled AuNPs provided a large surface area and multiple binding sites within a few steps of modification and microporous structures of multilayered MWCNTs to allow a high accessibility of target molecules. It was shown that the bulk refractive index (RI) sensitivity of these multilayered AuNPs (three-dimensional chip) appeared to be 5.6 times better than that of a monolayer of AuNPs on a glass chip (two-dimensional chip). The three-dimensional chips were further used for a biomolecular binding study, showing a detection limit as low as 0.5 nM for streptavidin and 3.33 nM for anti-human serum albumin (HSA), both of which were approximately 20 times higher than the sensitivity of the two-dimensional chips.

LSPR biomolecular assay with high sensitivity induced by aptamer-antigen-antibody sandwich complex.

Herein we demonstrate a sensitive approach for protein detection based on peak shifts of localized surface plasmon resonance (LSPR) induced by aptamer-antigen-antibody sandwich structures. The applicability of the proposed method is demonstrated using human α-thrombin as a model analyte. While the binding of thrombin to its specific receptor, thrombin binding aptamer (TBA) modified on Au nanorods (AuNRs), causes a measurable LSPR shift, a subsequent binding of an anti-thrombin antibody to the captured thrombin can exhibit a nearly 150% amplification in the LSPR response. This enhanced signal essentially leads to an improvement of limit of detection (LOD) by more than one order of magnitude. In addition, the use of TBA as thrombin recognition units makes the biosensor reusable. The feasibility of the proposed method was further exploited by the detection of thrombin in human serum, opening the possibility of a real application for diagnostics and medical investigations.

Surface-Enhanced Electrochemiluminescence of Ru@SiO2 for Ultrasensitive Detection of Carcinoembryonic Antigen

Carcinoembryonic antigen (CEA) is recognized as a disease biomarker to reflect the existence of various cancers and tumors in the human body. Sensitive detection of CEA in body fluid is valuable for clinical diagnosis and treatment assessment of cancers. Herein, we present a new approach for ultrasensitive determination of CEA in human serum based on localized surface plasmon resonance (LSPR) enhanced electrochemiluminescence (ECL) of Ru(bpy)3(2+). In this surface-enhanced ECL (SEECL) sensing scheme, Ru(bpy)3(2+)-doped SiO2 nanoparticles (Ru@SiO2) act as ECL luminophores, and AuNPs are used as LSPR source to enhance the ECL signal. Two different kinds of aptamers specific to CEA are modified on the surface of Ru@SiO2 and AuNPs, respectively. In the presence of CEA, a multilayer of Ru@SiO2-AuNPs nanoarchitectures would be formed. Our investigation reveals that the ECL signal of Ru@SiO2 can be effectively enhanced by AuNPs. One layer of Ru@SiO2-AuNPs nanoarchitectures would generate about 3-fold ECL enhancement compared with the ECL of the nanoarchitectures without the presence of AuNPs. As much as 30-fold ECL enhancement could be obtained by a multilayer of Ru@SiO2-AuNPs nanoarchitectures. Under the optimal conditions, a detection limit of 1.52 × 10(-6) ng/mL of CEA in human serum was achieved. To the best of our knowledge, CEA assays with such a low LOD have never been reported for an ECL sensor.

Highly stable and sensitive glucose biosensor based on covalently assembled high density Au nanostructures

We describe the development of a highly stable and sensitive glucose biosensor based on the nanohybrid materials derived from gold nanoparticles (AuNPs) and multi-walled carbon nanotubes (MWCNT). The biosensing platform was developed by using layer-by-layer (LBL) self-assembly of the nanohybrid materials and the enzyme glucose oxidase (GOx). A high density of AuNPs and MWCNT nanocomposite materials were constructed by alternate self assembly of thiol functionalized MWCNTs and AuNPs, followed by chemisoption of GOx. The surface morphology of multilayered AuNPs/MWCNT structure was characterized by field emission-scanning electron microscope (FE-SEM), and the surface coverage of AuNPs was investigated by cyclic voltammetry (CV), showing that 5 layers of assembly achieves the maximum particle density on electrode. The immobilization of GOx was monitored by electrochemical impedance spectroscopy (EIS). CV and amperometry methods were used to study the electrochemical oxidation of glucose at physiological pH 7.4. The Au electrode modified with five layers of AuNPs/MWCNT composites and GOx exhibited an excellent electrocatalytic activity towards oxidation of glucose, which presents a wide liner range from 20 μM to 10 mM, with a sensitivity of 19.27 μA mM(-1) cm(-2). The detection limit of present modified electrode was found to be 2.3 μM (S/N=3). In addition, the resulting biosensor showed a faster amperometric current response (within 3 s) and low apparent Michaelis-Menten constant (K(m)(app)). Our present study shows that the high density of AuNPs decorated MWCNT is a promising nanohybrid material for the construction of enzyme based electrochemical biosensors.

Nanoarray-based biomolecular detection using individual Au nanoparticles with minimized localized surface plasmon resonance variations.

Here, we present a mean to expand the use of individual metallic nanoparticles to two-dimensional plasmonic nanoarrays. An optical detection platform to track down localized surface plasmon resonance (LSPR) signals of individual nanoparticles on substrates was built for the application of plasmonic nanoarrays. A pseudoimage of nanoparticles on a substrate was reconstructed from their scattering spectra obtained by scanning a user-defined area. The spectral and spatial resolutions of the system were also discussed in detail. Most importantly, we present a method to normalize the localized surface plasmon resonance from geometrically different nanoparticles. After normalization, plasmonic responses from different particles become highly consistent, creating well-fitted dose-response curves of both surrounding refractive index changes and receptor-analyte binding to the surface of individual nanoparticles. Finally, the proof-of-concept system for plasmonic nanoarray detection is demonstrated by the measurement of the aptamer-thrombin binding event.

Highly Selective and Sensitive Electrochemiluminescence Biosensor for p53 DNA Sequence Based on Nicking Endonuclease Assisted Target Recycling and Hyperbranched Rolling Circle Amplification

An ultrasensitive and specific electrochemiluminescence (ECL) biosensor has been designed for the p53 DNA sequence, which is based on cascade signal amplification of nicking endonuclease assisted target recycling and hyperbranched rolling circle amplification (HRCA). First of all, biotin modified hairpin capture DNA (HP) probe was immobilized on the surface of streptavidin magnespheres paramagnetic particles (PMPs). Target DNA hybridized with the loop portion of the HP probe, therefore unfolding HP to form a double-stranded DNA (dsDNA) containing the specific nicking site of the nicking endonuclease. Then, the nicking endonuclease recognized the specific nicking site and cleaved the HP into two pieces, liberating target DNA and the complementary sequence piece for the padlock probe. The intact target DNA would initiate the next cycle of hybridization and cleavage, thereby releasing multiple complementary sequences for the padlock probes. The liberated complementary sequences hybridized with the padlock probes, subsequently inducing the HRCA reaction and generating numerous dsDNA segments. Herein, Ru(phen)3(2+) was embedded into dsDNA and worked as ECL signal reporter. The reaction products were eventually pretreated by dialysis tube with the cutoff membrane to remove the residual Ru(phen)3(2+) in the solution for the following ECL measurements. Using this cascade amplification strategy, an ultrasensitive p53 DNA sequence detection method was developed with a wide linear range from 0.05 to 100 fM and a low detection limit of 0.02 fM. Moreover, this cascade amplified ECL biosensor had specific recognition capacity for noncomplementary and single- and double-base mismatched DNA. The proposed ECL biosensor might have a great potential in biomedical research and clinic analysis.

Electrochemiluminescence biosensor for ultrasensitive determination of ochratoxin A in corn samples based on aptamer and hyperbranched rolling circle amplification

Determination of ochratoxin A (OTA) is highly important for food safety control. In this study, a signal-on electrochemiluminescence (ECL) biosensor which combined the characteristics of high efficiency of hyperbranched rolling circle amplification (HRCA) and high selectivity of aptamer was developed for OTA determination. The capture probe DNA (CDNA) was firstly immobilized on the gold electrode surface through Au-S interaction, then the OTA aptamer was modified on the electrode surface through hybridization with CDNA. Since OTA can competitively bind with the aptamer due to their high affinity, which would induce the releasing of aptamer from the electrode surface. Subsequently, the free CDNA on the electrode surface can hybridize with the padlock probe and induce HRCA reaction subsequently. Thus, the HRCA products which contained large amount of double-stranded DNA (dsDNA) fragments can be accumulated on the electrode surface. Since Ru(phen)3(2+) can intercalate into the groove of dsDNA and acts as ECL indicator, high ECL intensity can be detected from the electrode surface. The enhanced ECL intensity has a linear relationship with OTA in the range of 0.05-500 pg/mL with a correlation coefficient of 0.9957, and the limit of detection (LOD) was 0.02 pg/mL. The developed biosensor has been applied to determine OTA concentration in the corn samples with satisfied results.

Solid-phase colorimetric sensor based on gold nanoparticle-loaded polymer brushes: lead detection as a case study.

We introduce a novel solid-phase colorimetric sensor facilely fabricated by loading unmodified gold nanoparticles into poly(oligo(ethylene glycol)methacrylate) (POEGMA) brushes grown on glass. Our work reports the first synergistic combination of metallic nanoparticles acting as a colorimetric sensing module with a nonfouling polymer matrix acting both as a nonrigid scaffold and a screen to reduce interference from nontarget molecules. In addition, as the nanocomposite is formed on a transparent substrate, solid-phase detection can be performed in the same manner as in the solution-phase. We demonstrate the use of this unique platform for label-free lead detection based on the release of gold nanoparticles from the polymer brush upon exposure to lead ions. An ultralow limit-of-detection of 25 pM (S/N = 3) and a dynamic range of 100 pM to 100 nM (R(2) = 0.987) are achieved. Furthermore, the detection is up to 1000-fold more selective to lead over other common heavy metal ions.