Long-range fluorescence quenching by gold nanoparticles in a sandwich immunoassay for cardiac troponin T

Single-Molecule Detection of Polynucleotide Kinase Based on Phosphorylation-Directed Recovery of Fluorescence Quenched by Au Nanoparticles

5'-Polynucleotide kinase such as T4 polynucleotide kinase (T4 PNK) may catalyze the phosphorylation of 5'-hydroxyl termini in nucleic acids, playing a crucial role in DNA replication, DNA recombination, and DNA damage repair. Here, we demonstrate for the first time single-molecule detection of PNK based on phosphorylation-directed recovery of fluorescence quenched by Au nanoparticle (AuNP) in combination with lambda exonuclease-mediated cleavage reaction. In the presence of PNK, the γ-phosphate group from adenosine triphosphate (ATP) is transferred to 5'-hydroxyl terminus, resulting in 5'-phosphorylation of the hairpin probe. The phosphorylated hairpin probes may function as the substrates of lambda exonuclease and enable the removal of 5' mononucleotides from the stem, leading to the unfolding of hairpin structure and the formation of binding probes. The resultant binding probes may specifically hybridize with the AuNP-modified capture probes, forming double-strand DNA (dsDNA) duplexes with 5'-phosphate groups as the substrates of lambda exonuclease and subsequently leading to the cleavage of capture probes and the liberation of Cy5 molecules and the binding probes. The released binding probes may further hybridize with new capture probes, inducing cycles of digestion-release-hybridization and consequently the release of numerous Cy5 molecules. Through simply monitoring Cy5 molecules with total internal reflection fluorescence (TIRF)-based imaging, PNK activity can be quantitatively measured. This assay is very sensitive with a limit of detection of 9.77 × 10^-8 U/μL, and it may be further used to screen the PNK inhibitors and measure PNK in cancer cell extracts.

Novel Pyrimidine Tagged Silver Nanoparticle Based Fluorescent Immunoassay for the Detection of Pseudomonas aeruginosa

A simple pyrimidine-based fluorescent probe (R)-4-(anthracen-9-yl)-6- (naphthalen-1-yl)-1,6-dihydropyrimidine-2-amine (ANDPA) was synthesized through the greener one pot reaction and characterized by IR, NMR, and ESI-Mass. Glucose stabilized silver nanoparticles (Glu-AgNPs) were also synthesized and characterized using UV, IR, XRD, SEM, and TEM. When ANDPA was tagged with Glu-AgNPs, the fluorescent intensity of ANDPA decreased drastically. When the monoclonal antibody (Ab) [immunoglobulin G (IgG)] of Pseudomonas aeruginosa (PA) was attached with ANDPA/Glu-AgNPs, the original intensity of the probe was recovered with minimal enhancement at 446 nm. On further attachment of PA with ANDPA/Glu-AgNPs/PA, the fluorescence intensity of the probe was enhanced obviously at 446 nm with red shift. This phenomenon was further supported by SEM and TEM. The linear range of detection is from 8 to 10^-1 CFU/mL, and LOD is 1.5 CFU/mL. The immunosensor was successfully demonstrated to detect Pseudomonas aeruginosa in water, soil, and food products like milk, sugar cane, and orange juices.

Assessing the Intracellular Integrity of Phosphine-Stabilized Ultrasmall Cytotoxic Gold Nanoparticles Enabled by Fluorescence Labeling

As the size of nanoparticles (NPs) is in the range of biological molecules and subcellular structures, they provide new perspectives in biomedicine. This work presents studies concerning the cellular uptake and distribution of phosphine-stabilized cytotoxic 1.4 nm sized AuNPs and their probable degradation during this process. Therefore, ultrasmall phosphine-stabilized AuNPs are modified by linking a fluorophore covalently to the ligand shell. Monitoring the fluorescence on a cellular level by means of flow cytometry and confocal laser scanning microscopy allows determining the fate of the ligand shell during AuNP cell internalization, due to the fact that the fluorescence of a fluorophore bound near to the AuNP surface is quenched. Cell fractionation is conducted in order to quantify the AuNP content at the cell membrane, in the cytoplasm, and the cell nucleus. The incubation of cells with the fluorophore-modified AuNPs reveals a partial loss of the ligand shell upon AuNP cell interaction, evident by the emerging fluorescence signal. This loss is the precondition to unfold high AuNP cytotoxicity. Together with their significantly different biodistribution and enhanced circulation times compared to larger AuNPs, the findings demonstrate the high potential of ultrasmall AuNPs for drug development or therapy.

A Dual-Label Time-Resolved Fluorescence Immunoassay for the Simultaneous Determination of Cardiac Troponin T and Myoglobin

The aim of this study was to establish a dual-label time-resolved fluorescence immunoassay (TRFIA) for the simultaneous determination of cardiac troponin T (cTnT) and myoglobin (MYO) for the early diagnosis of acute myocardial infarction. The sandwich immunoassay was used to detect the concentration of cTnT and MYO in serum. cTnT and MYO in serum were captured by anti-cTnT and anti-MYO antibodies immobilized on microtiter wells and then banded together with another anti-cTnT and anti-MYO labeled with europium(III) Sm³⁺ and samarium(III) Eu³⁺ chelates, followed by fluorescence measurement using time-resolved fluorometry. The performance of this TRFIA was evaluated using the clinical serum and compared with the commercial assays. The linear correlation coefficients ( R²) of the cTnT and MYO standard curves were 0.9993 and 0.9995, respectively. The sensitivity for cTnT detection was 2.21 pg/mL (linear dynamic range was 3.24-963.71 pg/mL), and the average recovery was 100.57%. The sensitivity for MYO detection was 3.24 ng/mL (linear dynamic range was 4.95-976.85 ng/mL), and the average recovery was 99.79%. High correlation coefficients ( R²) were obtained between the commercial assays and dual-label TRFIA ( R² = 0.999). The present dual-label TRFIA has high sensitivity, specificity, and accuracy in clinical sample analysis. It is a good alternative to the single-label diagnostic methods.

Design of a novel plasmonic nanoconjugated analytical tool for ultrasensitive antigen quantification

To date, while various diagnostic approaches for antigen detection have been proposed, most are too expensive, lengthy and limited in sensitivity for clinical use. Nanoparticle systems with unique material properties, however, circumvent these problems and offer improved accuracy and sensitivity over current methods like the enzyme-linked immunosorbent assay (ELISA). Herein, we present a novel functionalization strategy of plasmonic nanoparticle probes capable of specific quantification of antigens directly in clinical samples. A nanoconjugation strategy that allows one to perform an intensity depletion immuno-linked assay (IDILA), involving specific antibodies that target the antigen of interest was designed to obtain a calibration curve and achieve the quantification of the antigen in clinical samples in the same experiment using a microplate reader (i.e., an UV-vis spectrophotometer). Finally, the IDILA methodology allowed specific detection of various clinically relevant antigens, with significantly improved sensitivity over the ELISA. Furthermore, the assay was shown to be robust, reliable, cheap and rapid, diagnosing antigens in clinical serum samples within 2 hours.

Hairpin DNA-functionalized gold nanorods for mRNA detection in homogenous solution

We report a fluorescent probe for mRNA detection. It consists of a gold nanorod (GNR) functionalized with fluorophore-labeled hairpin oligonucleotides (hpDNA) that are complementary to the mRNA of a target gene. This nanoprobe was found to be sensitive to a complementary oligonucleotide, as indicated by significant changes in both fluorescence intensity and lifetime. The influence of the surface density of hpDNA on the performance of this nanoprobe was investigated, suggesting that high hybridization efficiency could be achieved at a relatively low surface loading density of hpDNA. However, steady-state fluorescence spectroscopy revealed better overall performance, in terms of sensitivity and detection range, for nanoprobes with higher hairpin coverage. Time-resolved fluorescence lifetime spectroscopy revealed significant lifetime changes of the fluorophore upon hybridization of hpDNA with targets, providing further insight on the hybridization kinetics of the probe as well as the quenching efficiency of GNRs.

Short-range ordered photonic structures of lamellae-forming diblock copolymers for excitation-regulated fluorescence enhancement

Photonic crystals can be represented by periodic nanostructures with alternating refractive indices, which create artificial stop bands with the appearance of colors. In this regard, nanodomains of block copolymers and the corresponding structural colors have been intensively studied in the past. However, the practical application of photonic crystals of block copolymers has been limited to a large degree because of the presence of large defects and grain boundaries in the nanodomains of block copolymers. The present study focuses on the alternative opportunity of short-range ordered nanodomains of block copolymers for fluorescence enhancement, which also has a direct relevance to the development of fluorescence sensors or detectors. The enhancement mechanism was found to be interconnected with the excitation process rather than the alternation of the decay kinetics. In particular, we demonstrate that randomly oriented, but regular grains of lamellae of polystyrene-block-polyisoprene, PS-b-PI, diblock copolymers and their blend with PS homopolymers can behave as Bragg mirrors to induce multiple reflections of the excitation source inside the photonic structures. This process in turn significantly increases the effective absorption of the given fluorophores inside the polymeric photonic structures to amplify the fluorescence signal.

Gold nanoparticle synthesis coupled to fluorescence turn-on for sensitive detection of formaldehyde using formaldehyde dehydrogenase

Ultrasensitive detection of formaldehyde by coupling enzyme activity with GNP synthesis.

Bioapplications and biotechnologies of upconversion nanoparticle-based nanosensors

Upconversion nanoparticles (UCNPs), which can emit ultraviolet/visible (UV/Vis) light under near-infrared (NIR) excitation, are regarded as a new generation of nanoprobes because of their unique optical properties, including a virtually zero auto-fluorescence background for the improved signal-to-noise ratio, narrow emission bandwidths and high resistance to photo-bleaching.

Nanoparticle-Based and Bioengineered Probes and Sensors to Detect Physiological and Pathological Biomarkers in Neural Cells

Nanotechnology, a rapidly evolving field, provides simple and practical tools to investigate the nervous system in health and disease. Among these tools are nanoparticle-based probes and sensors that detect biochemical and physiological properties of neurons and glia, and generate signals proportionate to physical, chemical, and/or electrical changes in these cells. In this context, quantum dots (QDs), carbon-based structures (C-dots, grapheme, and nanodiamonds) and gold nanoparticles are the most commonly used nanostructures. They can detect and measure enzymatic activities of proteases (metalloproteinases, caspases), ions, metabolites, and other biomolecules under physiological or pathological conditions in neural cells. Here, we provide some examples of nanoparticle-based and genetically engineered probes and sensors that are used to reveal changes in protease activities and calcium ion concentrations. Although significant progress in developing these tools has been made for probing neural cells, several challenges remain. We review many common hurdles in sensor development, while highlighting certain advances. In the end, we propose some future directions and ideas for developing practical tools for neural cell investigations, based on the maxim "Measure what is measurable, and make measurable what is not so" (Galileo Galilei).

Characteristics of localized surface plasmons excited on mixed monolayers composed of self-assembled Ag and Au nanoparticles

The fundamental characteristics of localized surface plasmon resonance (LSPR) excited on mixed monolayers composed of self-assembled Ag and Au nanoparticles (AgNPs and AuNPs, respectively) were investigated. Mixed monolayered films were fabricated at the air-water interface at different mixing ratios. The films retained their phase-segregated morphologies in which AuNPs formed several 10 to 100 nm island domains in a homogeneous AgNP matrix phase. The LSPR bands originating from the self-assembled domains shifted to longer wavelengths as the domain size increased, as predicted by a finite-difference time-domain (FDTD) simulation. The FDTD simulation also revealed that even an alternating-lattice-structured two-dimensional (2D) AgNP/AuNP film retained two isolated LSPR bands, revealing that the plasmon resonances excited on each particle did not couple even in a continuous 2D sheet, unlike in the homologous NP system. The fluorescence quenching test of Cy3 and Cy5 dyes confirmed that the independent functions of AuNPs and AgNPs remained in the mixed films, whereas the AuNPs exhibited significantly higher quenching efficiency for the Cy3 dye compared with AgNPs due to the overlap of the excitation/emission bands of the dyes with the AuNP LSPR band. Various applications can be considered using this nanoheterostructured plasmonic assembly to excite spatially designed, high-density LSPR on macroscopic surfaces.

Gold Nanocluster-Assisted Fluorescent Detection for Hydrogen Peroxide and Cholesterol Based on the Inner Filter Effect of Gold Nanoparticles

We developed a simple, sensitive inner filter effect (IFE)-based fluorescent assay for sensing H2O2 and cholesterol. In the process, poly(vinylpyrrolidone)-protected gold nanoparticles (PVP-AuNPs) and fluorescent BSA-protected gold nanoclusters (BSA-AuNCs) were used as an IFE absorber/fluorophore pair. PVP-AuNPs can be a powerful absorber to influence the emission of the fluorophore, BSA-AuNCs, in the IFE-based fluorescent assays. That is due to the high extinction coefficient of AuNPs and the complementary overlap between the surface plasmon resonance (SPR) absorption of PVP-AuNPs and the excitation of BSA-AuNCs. The PVP-Au seeds, produced by directly mixing PVP with HAuCl4, were able to catalyze H2O2 to enlarge AuNPs. The SPR absorption of PVP-AuNPs was enhanced with an increased concentration of H2O2 and, subsequently, induced significant fluorescence quenching of BSA-AuNCs. The IFE-based fluorescent assay enabled the detection of H2O2 and generation of H2O2 in the presence of O2/cholesterol and cholesterol oxidase (ChOx) by the fluorescence response of BSA-AuNCs. The present IFE-based approach can detect H2O2 ranging from 1 to 100 μM with a detection limit of 0.8 μM and cholesterol ranging from 1 to 100 μM with a detection limit of 1.4 μM.

Nanoscopic optical rulers beyond the FRET distance limit: fundamentals and applications

In the last few decades, Förster resonance energy transfer (FRET) based spectroscopy rulers have served as a key tool for the understanding of chemical and biochemical processes, even at the single molecule level. Since the FRET process originates from dipole-dipole interactions, the length scale of a FRET ruler is limited to a maximum of 10 nm. Recently, scientists have reported a nanomaterial based long-range optical ruler, where one can overcome the FRET optical ruler distance dependence limit, and which can be very useful for monitoring biological processes that occur across a greater distance than the 10 nm scale. Advancement of nanoscopic long range optical rulers in the last ten years indicate that, in addition to their long-range capability, their brightness, long lifetime, lack of blinking, and chemical stability make nanoparticle based rulers a good choice for long range optical probes. The current review discusses the basic concepts and unique light-focusing properties of plasmonic nanoparticles which are useful in the development of long range one dimensional to three dimensional optical rulers. In addition, to provide the readers with an overview of the exciting opportunities within this field, this review discusses the applications of long range rulers for monitoring biological and chemical processes. At the end, we conclude by speculating on the role of long range optical rulers in future scientific research and discuss possible problems, outlooks and future needs in the use of optical rulers for technological applications.

pH-Responsive assembly of metal nanoparticles and fluorescent dyes by diblock copolymer micelles

Hybrid assemblies consisting of metal nanoparticles (NPs) and fluorophores are quite interesting because the intrinsic properties of fluorophores can be engineered in the assembled structure. In this regard, we utilized the self-segregation properties of block copolymer micelles to organize metal NPs and fluorophores simultaneously in a specific arrangement. From the viewpoint of assembly methods, we first encapsulated Au NPs in the PS cores of polystyrene-block-poly(acrylic acid) (PS-PAA) micelles. Then, positively charged fluorescent dyes of rhodamine 123 (R123) were bound to the negatively charged PAA coronas by electrostatic interactions. Since carboxylic acid in the PAA block is a weak acid, the degree of R123 binding to PS-PAA micelles can be adjusted by varying the pH of the solution. Therefore, by changing the pH, we were able to control the assembly and disassembly of R123 molecules to PS-PAA micelles and the corresponding change in the fluorescence signal.

Surface plasmon enhanced energy transfer between gold nanorods and fluorophores: application to endocytosis study and RNA detection

Previously we have demonstrated surface plasmon enhanced energy transfer between fluorophores and gold nanorods under two-photon excitation using fluorescence lifetime imaging microscopy (FLIM) in both solution and intracellular phases. These studies demonstrated that gold nanoparticle-dye energy transfer combinations are appealing, not only in Förster resonance energy transfer (FRET) imaging, but also energy transfer-based fluorescence lifetime sensing of bio-analytes. Here, we apply this approach to study the internalization of gold nanorods (GNRs) in HeLa cells using the early endosome labeling marker GFP. The observed energy transfer between GFP and the GNRs indicates the involvement of endocytosis in GNR uptake. Moreover, a novel nanoprobe based on oligonucleotide functionalized gold nanorods for nucleic acid sensing via dye-GNRs energy transfer is demonstrated, potentially opening up new possibilities in cancer diagnosis and prognosis. The influence of oligonucleotide design on such nanoprobe performance was studied for the first time using time-resolved fluorescence spectroscopy, bringing new insights to the optimization of the nanoprobe.

Nanomaterial-based biosensors using dual transducing elements for solution phase detection

Biosensors incorporating nanomaterials have demonstrated superior performance compared to their conventional counterparts. Most reported sensors use nanomaterials as a single transducer of signals, while biosensor designs using dual transducing elements have emerged as new approaches to further improve overall sensing performance. This review focuses on recent developments in nanomaterial-based biosensors using dual transducing elements for solution phase detection. The review begins with a brief introduction of the commonly used nanomaterial transducers suitable for designing dual element sensors, including quantum dots, metal nanoparticles, upconversion nanoparticles, graphene, graphene oxide, carbon nanotubes, and carbon nanodots. This is followed by the presentation of the four basic design principles, namely Förster Resonance Energy Transfer (FRET), Amplified Fluorescence Polarization (AFP), Bio-barcode Assay (BCA) and Chemiluminescence (CL), involving either two kinds of nanomaterials, or one nanomaterial and an organic luminescent agent (e.g. organic dyes, luminescent polymers) as dual transducers. Biomolecular and chemical analytes or biological interactions are detected by their control of the assembly and disassembly of the two transducing elements that change the distance between them, the size of the fluorophore-containing composite, or the catalytic properties of the nanomaterial transducers, among other property changes. Comparative discussions on their respective design rules and overall performances are presented afterwards. Compared with the single transducer biosensor design, such a dual-transducer configuration exhibits much enhanced flexibility and design versatility, allowing biosensors to be more specifically devised for various purposes. The review ends by highlighting some of the further development opportunities in this field.

Nanoparticle-based caspase sensors

Recent advances in nanotechnology have provided new tools for measuring enzymatic activities that are relevant for the assessment of physiological and pathological processes. Caspases, the enzymes intimately linked with cell death and inflammation, are cysteine-dependent aspartate-directed proteases. The measurement of caspase activity requires assays that can provide data with specificity, precision and sensitivity. Several nanoparticle-based assays are now beginning to emerge. This article will first provide a brief discussion of conventional methods of measuring caspase activity and their limitations, followed by an overview of the advantages and limitations of nanoparticle-based strategies for sensing caspase enzymatic activity in vitro and in vivo.

Prevention of doxorubicin sorptive losses in drug delivery studies using polyethylene glycol

Polyethylene glycol enhances the accuracy of drug delivery system evaluations by preventing sorptive losses of hydrophobic drugs to plastic reaction vessels.

Tailoring of optical properties of fluorescein using green synthesized gold nanoparticles

The anisotropy in plasmonic field makes star shaped particles as an optical ruler that can probe larger distances as compared to spherical gold nanoparticles, for which dipoles are parallel to the surface act as more efficient quencher for fluorescein dye.

Nanoparticle based fluorescence resonance energy transfer (FRET) for biosensing applications

Nanoparticle based FRET assays have higher energy transfer efficiency and better performance compared with traditional organic fluorophore based FRET assays.

Spectroscopic studies of 1,4-dimethoxy-2,3-dimethylanthracene-9,10-dione on plasmonic silver nanoparticles

Silver nanoparticles (Ag NPs) of different sizes from 7nm to 22nm have been prepared by simple Dirk and Charles chemical method and characterized using UV-vis spectroscopy and high resolution transmission electron microscopy (HRTEM). Fluorescence quenching of 1,4-dimethoxy-2,3-dimethylanthracene-9,10-dione (DMDMAD) by silver nanoparticles has been investigated by fluorescence spectroscopy to understand the role of quenching mechanism. Furthermore, the intensity of DMDMAD fluorescence emission peak decreases with decrease in the size of the Ag NPs. The fluorescence quenching rate constant and association constant for above system were determined using Stern-Volmer and Benesi-Hildebrand plots. The mechanism of DMDMAD fluorescence quenched by Ag NPs was discussed according to the Stern-Volmer equation. It has been observed that the quenching due to Ag NPs proceeds via dynamic quenching process. The distance between DMDMAD (donor) to Ag NPs (acceptor) and the critical energy transfer distance were estimated based on the Förster Resonance Energy Transfer (FRET) theory.

Re-evaluation of biotin-streptavidin conjugation in Förster resonance energy transfer applications

Bioaffinity conjugation between streptavidin (SA) and biotin has been widely used to link donors and acceptors for investigating the distance-dependent Förster resonance energy transfer (FRET). When studying a commonly used FRET system of (QD-SA)-(biotin-DNA-dye) [donor: quantum dot (QD); acceptor: small organic fluorescent dye; and linker: deoxyribose nucleic acid (DNA) molecule via SA-biotin conjugation], however, a contradictory finding was recently reported in the literature. It was found that the FRET lost its dependence on the number of DNA base pairs when using a phosphate-buffered saline (PBS) solution. We found that the conflicted results were caused by the ionic strength of the adopted buffer solutions. Our results suggest that the dependent FRET on the number of DNA bases is favorable in a low-ionic-strength buffer, whereas in relatively high-ionic-strength buffers, the FRET loses the DNA length dependence. We propose that the independence is mainly caused by the conformational change of DNA molecules from a stretched to a coiled mode when the cations in the high-ionic-strength buffer neutralize the negatively charged backbone of DNA molecules, thereby bringing the acceptors close to the donors.

Revisiting 30 years of biofunctionalization and surface chemistry of inorganic nanoparticles for nanomedicine

In the last 30 years we have assisted to a massive advance of nanomaterials in material science. Nanomaterials and structures, in addition to their small size, have properties that differ from those of larger bulk materials, making them ideal for a host of novel applications. The spread of nanotechnology in the last years has been due to the improvement of synthesis and characterization methods on the nanoscale, a field rich in new physical phenomena and synthetic opportunities. In fact, the development of functional nanoparticles has progressed exponentially over the past two decades. This work aims to extensively review 30 years of different strategies of surface modification and functionalization of noble metal (gold) nanoparticles, magnetic nanocrystals and semiconductor nanoparticles, such as quantum dots. The aim of this review is not only to provide in-depth insights into the different biofunctionalization and characterization methods, but also to give an overview of possibilities and limitations of the available nanoparticles.

Förster resonance energy transfer studies of luminescent gold nanoparticles functionalized with ruthenium(II) and rhenium(I) complexes: modulation via esterase hydrolysis

A number of ruthenium(II) and rhenium(I) bipyridine complexes functionalized with lipoic acid moieties have been synthesized and characterized. Functionalization of gold nanoparticles with these chromophoric ruthenium(II) and rhenium(I) complexes has resulted in interesting supramolecular assemblies with Förster resonance energy transfer (FRET) properties that could be modulated via esterase hydrolysis. The luminescence of the metal complex chromophores was turned on upon cleavage of the ester bond linkage by esterase to reduce the efficiency of FRET quenching. The prepared nanoassembly conjugates have been characterized by transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), UV-visible spectroscopy, and emission spectroscopy. The quenching mechanism has also been studied by transient absorption and time-resolved emission decay measurements. The FRET efficiencies were found to vary with the nature of the chromophores and the length of the spacer between the donor (transition metal complexes) and the acceptor (gold nanoparticles).

Understanding the self-assembly of proteins onto gold nanoparticles and quantum dots driven by metal-histidine coordination

Coupling of polyhistidine-appended biomolecules to inorganic nanocrystals driven by metal-affinity interactions is a greatly promising strategy to form hybrid bioconjugates. It is simple to implement and can take advantage of the fact that polyhistidine-appended proteins and peptides are routinely prepared using well established molecular engineering techniques. A few groups have shown its effectiveness for coupling proteins onto Zn- or Cd-rich semiconductor quantum dots (QDs). Expanding this conjugation scheme to other metal-rich nanoparticles (NPs) such as AuNPs would be of great interest to researchers actively seeking effective means for interfacing nanostructured materials with biology. In this report, we investigated the metal-affinity driven self-assembly between AuNPs and two engineered proteins, a His7-appended maltose binding protein (MBP-His) and a fluorescent His6-terminated mCherry protein. In particular, we investigated the influence of the capping ligand affinity to the nanoparticle surface, its density, and its lateral extension on the AuNP-protein self-assembly. Affinity gel chromatography was used to test the AuNP-MPB-His7 self-assembly, while NP-to-mCherry-His6 binding was evaluated using fluorescence measurements. We also assessed the kinetics of the self-assembly between AuNPs and proteins in solution, using time-dependent changes in the energy transfer quenching of mCherry fluorescent proteins as they immobilize onto the AuNP surface. This allowed determination of the dissociation rate constant, Kd(-1) ∼ 1-5 nM. Furthermore, a close comparison of the protein self-assembly onto AuNPs or QDs provided additional insights into which parameters control the interactions between imidazoles and metal ions in these systems.

Theoretical analysis of a magnetophoresis-diffusion T-sensor immunoassay

We present the analytical investigation of a microfluidic homogeneous competitive immunoassay that incorporates antibody-conjugated superparamagnetic nanoparticles and magnetophoretic transport to enhance the limits of detection and dynamic range. The analytical model considers the advective, diffusive, and magnetophoretic transport of the antibody-coated nanoparticles relative to the labeled and sample antigens of interest in a T-sensor configuration. The magnetophoresis-diffusion immunoassay identified clear improvements to the assay response and reductions to the limit of detection for increased magnetophoretic velocities and larger nanoparticles. The externally applied magnetophoretic transport enriched the antibody-antigen accumulation region, while larger nanoparticles led to decreased diffusive peak broadening. The integration of nanoparticles to the diffusion immunoassay (NP-DIA) demonstrated an approximately 3-fold improvement to the limit of detection of the basic antibody/antigen system, while the integration of superparamagnetic nanoparticles and magnetophoretic transport (MIA) established an order of magnitude improvement in sensitivity as well as means to greatly reduce response time. The implementation of an external magnetic force enabled the detectable antigen size spectrum to extend from small molecules i.e., 10's Da to 100's Da, up to large proteins and macromolecules, i.e., 50 kDa to 150 kDa, for a single class of binding species, i.e., superparamagnetic nanoparticle. This investigation provides guidelines for the design and development of a magnetophoresis-diffusion T-sensor immunoassay, and clearly identifies the regimes for optimal operation.

Accelerated colorimetric immunosensing using surface-modified porous monoliths and gold nanoparticles

A rapid and sensitive immunoassay platform integrating polymerized monoliths and gold nanoparticles (AuNPs) has been developed. The porous monoliths are photopolymerized in situ within a silica capillary and serve as solid support for high-mass transport and high-density capture antibody immobilization to create a shorter diffusion length for antibody-antigen interactions, resulting in a rapid assay and low reagent consumption. AuNPs are modified with detection antibodies and are utilized as signals for colorimetric immunoassays without the need for enzyme, substrate and sophisticated equipment for quantitative measurements. This platform has been verified by performing a human IgG sandwich immunoassay with a detection limit of 0.1 ng ml^-1. In addition, a single assay can be completed in 1 h, which is more efficient than traditional immunoassays that require several hours to complete.

High throughput and high yield nanofabrication of precisely designed gold nanohole arrays for fluorescence enhanced detection of biomarkers

Fluorescence excitation enhancement by plasmonic nanostructures such as gold nanohole arrays has been a hot topic in biosensing and bioimaging in recent years. However, the high throughput and high yield fabrication of precisely designed metal nanostructures for optimized fluorescence excitation remains a challenge. Our work is the first report combining nanopattern nickel mould fabrication and UV imprinting for gold nanostructure mass fabrication in high yield. We report our successful gold nanohole array mass fabrication on a 4'' glass wafer, by first fabricating a high fidelity nickel mould, then using the mould for UV nanoimprinting on a polymer coated on the glass, evaporating the gold film on the glass wafer, and lifting off the polymer to obtain a gold nanohole array on the glass. Our optimized process for wafer fabrication can achieve almost 100% yield from nanoimprinting to gold lift-off, while the fabricated nickel mould has >70% defect-free area with the rest having a few scattered defects. In our work, the size and pitch of the gold nanohole array are designed to enhance the fluorescent dye Alexa 647. When the fabricated gold nanohole array is used for prostate specific antigen (PSA) detection by establishing a sandwiched fluorescence assay on the gold surface, a detection limit of 100 pg ml(-1) is achieved, while with a same thickness of gold film, only 1 ng ml(-1) is detected.