Nanomaterials for ultrasensitive protein detection

Target-programmed and autonomous proximity binding aptasensor for amplified electronic detection of thrombin

The development of sensitive and simple approaches capable of monitoring trace amounts of protein biomarkers is appealing for disease diagnosis and treatment. Towards this end, we have developed an electrochemical sensing platform for sensitive and simple detection of protein biomarkers by using thrombin as the model target molecules via a target-programmed proximity binding amplification approach. The binding of thrombin to the aptamer sequences in the partial dsDNA duplex probes induces the release of the ssDNA trigger strands, which catalyze subsequent assembly formation of many methylene blue (MB)-tagged proximate DNA motifs with the presence of the DNA fuel strands through cascaded toehold-mediated strand displacement reactions. Due to the proximity-binding effect, these MB-tagged proximate DNA motifs anneal with the capture probes on the sensor surface with significantly enhanced stability against the corresponding single component counterpart, thereby pulling the MB tags close to the sensor surface and generating substantially amplified signal responses for sensitive determination of thrombin down to 23.6 pM. In addition, such aptasensor can specifically discriminate thrombin from other interference proteins, and can also be utilized to monitor thrombin in diluted serum samples, demonstrating its great potential for sensitive determination of proteins for early disease diagnosis.

A Self-Contained Chemiluminescent Lateral Flow Assay for Point-of-Care Testing

Immunoassays whose readouts rely on chemiluminescence are increasingly useful for a broad range of analytical applications, but they are rarely made into point-of-care (POC) format because of the complex reagents required (some reagents have to be stored in low temperatures, and some reagents have to be freshly made right before the assay). This study reports a self-contained chemiluminescent lateral flow assay (CLFA), which prestores all necessary reagents. This CLFA contains three parts: the normal lateral flow assay (LFA) strip, the chemiluminescence substrate pad, and the polycarbonate (PC) holder. On the LFA strip, we simultaneously labeled horseradish peroxidase (HRP) and antibody on the gold nanoparticles (AuNPs) for the conjugate pad. For the substrate pad, we used sodium perborate as the oxidant and lyophilized the chemiluminescence substrate on the glass fiber, which allows long-term storage. After the transfer of substrate from the substrate pad to the nitrocellulose (NC) membrane, we captured the chemiluminescence signal for the quantification of the targets. The HRP on the AuNPs can amplify the chemiluminescence signal efficiently. We used this CLFA system to detect both macromolecules and small molecules successfully. This self-contained and easily processable device is exceedingly appropriate for rapid detection and is a convenient platform for POC testing.

Ag+ -Gated Surface Chemistry of Gold Nanoparticles and Colorimetric Detection of Acetylcholinesterase

Chemical regulation of enzyme-mimic activity of nanomaterials is challenging because it requires a precise understanding of the surface chemistry and mechanism, and rationally designed applications. Herein, Ag+ -gated peroxidase activity is demonstrated by successfully modulating surface chemistry of cetyltrimethylammonium bromide-capped gold nanoparticles (CTAB-AuNPs). A surface blocking effect of long-chain molecules on surfaces of AuNPs that inhibit peroxidase activity of AuNPs is found. Ag+ ions can selectively bind on the surfaces of AuNPs and competitively destroy CTAB membrane forming Ag+ @CTAB-AuNPs complexes to result in enhanced peroxidase activity. Ag+ @CTAB-AuNPs show the highest peroxidase activity compared to similar-sized citrate-capped and ascorbic acid-capped AuNPs. Ag+ @CTAB-AuNPs can potentially develop into analyte-responsive systems and exhibit advantages in the optical sensing field. For example, the Ag+ @CTAB-AuNPs system shows an enhanced sensitivity and selectivity for acetylcholinesterase activity sensing compared to other methods.

Screening Small Metabolites from Cells as Multifunctional Coatings Simultaneously Improves Nanomaterial Biocompatibility and Functionality

Currently, nanomaterials face a dilemma due to their advantageous properties and potential risks to human health. Here, a strategy to improve both nanomaterial biocompatibility and functionality is established by screening small metabolites from cells as nanomaterial coatings. A metabolomics analysis of cells exposed to nanosilver (nAg) integrates volcano plots (t-tests and fold change analysis), partial least squares-discriminant analysis (PLS-DA), and significance analysis of microarrays (SAM) and identifies six metabolites (l-aspartic acid, l-malic acid, myoinositol, d-sorbitol, citric acid, and l-cysteine). The further analysis of cell viability, oxidative stress, and cell apoptosis reveals that d-sorbitol markedly reduces nAg cytotoxicity. Subsequently, small molecule loading, surface oxidation, and ionic release experiments support d-sorbitol as the optimal coating for nAg. Importantly, d-sorbitol loading improves the duration of the antibacterial activity of nAg against Escherichia coli and Staphylococcus aureus. The biocidal persistence of nAg-sorbitol is extended beyond 9 h, and the biocidal effects at 12 h are significantly higher than those of naked nAg. This work proposes a new strategy to improve the biocompatibility and functionality of nAg simultaneously by screening small metabolites from cells as nanomaterial functional coatings, a method that can be applied to mitigate the side effects of other nanomaterials.

"Three-in-one" Nanohybrids as Synergistic Nanoquenchers to Enhance No-Wash Fluorescence Biosensors for Ratiometric Detection of Cancer Biomarkers

Purpose: Early diagnosis of cancer enables extended survival and reduced symptoms. To this end, a "three-in-one" nanohybrid of MOF@AuNP@GO is designed as synergistic nanoquencher to develop a novel fluorescence biosensor for rapid and sensitive detection of cancer-related biomarkers. Methods: The ssDNA absorption affinities and fluorescence quenching abilities of the MOF@AuNP@GO were evaluated using FAM-labeled single-stranded DNA (ssDNA). Then, two specific dye-labeled ssDNA and aptamer probes were designed for the recognition of p53 gene and prostate specific antigen (PSA), respectively. Fluorescence spectra were recorded and ratiometric signal processing was performed. Results: The designed nanohybrids exhibit enhanced ssDNA binding affinities and fluorescence quenching abilities, which significantly decrease the background signal and increase the signal-to-noise (S/N) ratio, thus lowering the detection limit (LOD). Accordingly, with ratiometric measurement, this developed nanosensor can sensitively measure p53 gene and PSA with LODs of 0.005 nM and 0.01 ng mL-1, respectively. Besides, this method also displays excellent performances with respect to universality, multiplexed detection, specificity, and practicality in human serum. Conclusion: The designed MOF@AuNP@GO-based fluorescence biosensor can serve as a promising platform for washing-free, rapid and sensitive measurement of cancer biomarkers, making this method well-suited for point-of-care (POC) diagnosis.

Cascade Reaction-Mediated Assembly of Magnetic/Silver Nanoparticles for Amplified Magnetic Biosensing

Conventional magnetic relaxation switching (MRS) sensor suffers from its relatively low sensitivity when it comes to the analysis of trace small molecules in complicated samples. To meet this challenge, we develop a cascade reaction-mediated magnetic relaxation switching (CR-MRS) sensor, based on the assembly of silver nanoparticles (Ag NPs) and magnetic nanoparticles (MNPs) to improve the sensitivity of conventional MRS. The cascade reaction triggered by alkaline phosphatase generates ascorbic acid, which reduces Ag⁺ to Ag NPs that can assemble the initially dispersed MNPs to form magnetic/silver nanoassemblies, thus modulating the state of MNPs to result in the change of transverse relaxation time. The formed magnetic/silver nanoassemblies can greatly enhance the state change of MNPs (from dispersed to aggregated) and dramatically improve the sensitivity of traditional MRS sensor, which makes this CR-MRS sensor a promising platform for highly sensitive detection of small molecules in complicated samples.

Colorimetric Immunosensor by Aggregation of Photochemically Functionalized Gold Nanoparticles

A colorimetric immunosensor based on local surface plasmon resonance by gold nanoparticles is presented, and its application for the detection of human immunoglobulin G (IgG) is demonstrated. The color change of the colloidal solution is produced by nanoparticle aggregation, a process that can be tuned by the presence of the analyte once the nanoparticles are functionalized. In comparison to common functionalization techniques, the procedure described here is simpler, low-cost, and effective in binding antibodies upright on the gold surface. The dose-response curve is similar to that resulting in typical immunoassay platforms and is satisfactorily described by the proposed theoretical model. Human IgG at concentration levels of few hundreds of nanograms per milliliter can be detected by eyes within a few minutes, thereby making the colorimetric immunosensor proposed here a powerful tool in several areas, with urine test in medical diagnostics being the most immediate.

T1-Mediated Nanosensor for Immunoassay Based on an Activatable MnO2 Nanoassembly

Current magnetic relaxation switching (MRS) sensors for detection of trace targets in complex samples still suffer from limitations in terms of relatively low sensitivity and poor stability. To meet this challenge, we develop a longitudinal relaxation time (T₁)-based nanosensor by using Mn²⁺ released from the reduction of a MnO₂ nanoassembly that can induce the change of T₁, and thus can greatly improve the sensitivity and overcome the "hook effect" of conventional MRS. Through the specific interaction between antigen and the antibody-functionalized MnO₂ nanoassembly, the T₁ signal of Mn²⁺ released from the nanoassembly is quantitatively determined by the antigen, which allows for highly sensitive and straightforward detection of targets. This approach broadens the applicability of magnetic biosensors and has great potential for applications in early diagnosis of disease biomarkers.

DNA-Mediated rolling circle amplification for ultrasensitive detection of thrombin using MALDI-TOF mass spectrometry

A DNA-mediated rolling circle amplification (RCA) strategy was established for ultrasensitive and specific detection of thrombin via MALDI-TOF MS.

Recent progress in upconversion luminescence nanomaterials for biomedical applications

This review focuses on the biomedical applications of upconversion luminescence nanomaterials, including lanthanide-doped inorganic nanocrystals and TTA-based UCNPs.

A gold nanoparticle-based four-color proximity immunoassay for one-step, multiplexed detection of protein biomarkers using ribonuclease H signal amplification

A gold nanoparticle-based four-color fluorescence proximity immunoassay was developed for multiplexed analysis of protein biomarkers using ribonuclease H signal amplification.

Aptamer-functionalized magnetic and fluorescent nanospheres for one-step sensitive detection of thrombin

A one-step sandwich method is described for detecting proteins with magnetic nanospheres (MNs) and fluorescent nanospheres (FNs). Thrombin is selected as a model analyte to validate the method. Two DNA aptamers (Apt 29 and Apt 15 targeting two different exosites of thrombin) are chosen as recognition elements to modify MNs and FNs. The superparamagnetic MN-Apt 29 conjugate is used to separate and concentrate thrombin. The FN-Apt 15 conjugate encapsulates hundreds of fluorescent quantum dots and is used as reporter to provide a stable signal. Magnetic capture and fluorescence identification are performed simultaneously to form a sandwich complex (MN-Apt 29-thrombin-FN-Apt 15) for fluorescence determination (at excitation/emission wavelengths of 380/622 nm). The method is convenient, time saving, and gives a strong signal (compared to the two-step method where capture and identification are performed in two steps). The one-step method presented here is completed within 30 min and has a 3.5 ng·mL^-1 (97 pM) detection limit. The method is reproducible, has an intra-assay variability of 1.5%, and an inter-assay variability of 4.9%. Other serum proteins (HSA, CEA, PSA, and AFP) do not interfere. The method was also applied to analyze serum samples. Almost the same fluorescence intensity was measured when analyzing 1% serum samples (compared to buffer samples). Graphical abstract Magnetic nanospheres with excellent superparamagnetic property and fluorescent QD-based nanospheres were prepared and used in a one-step sensitive method for detecting thrombin. The method exhibits good reproducibility, high specificity, and good selectivity.

A Novel Detection Method of Human Serum Albumin Based on the Poly(Thymine)-Templated Copper Nanoparticles

In this work, we developed a facile fluorescence method for quantitative detection of human serum albumin (HSA) based on the inhibition of poly(thymine) (poly T)-templated copper nanoparticles (CuNPs) in the presence of HSA. Under normal circumstances, poly T-templated CuNPs can display strong fluorescence with excitation/emission peaks at 340/610 nm. However, in the presence of HSA, it will absorb cupric ion, which will prevent the formation of CuNPs. As a result, the fluorescence intensity will become obviously lower in the presence of HSA. The analyte HSA concentration had a proportional linear relationship with the fluorescence intensity of CuNPs. The detection limit for HSA was 8.2 × 10⁻⁸ mol·L⁻¹. Furthermore, it was also successfully employed to determine HSA in biological samples. Thus, this method has potential applications in point-of-care medical diagnosis and biomedical research.

In Situ Two-Step Photoreduced SERS Materials for On-Chip Single-Molecule Spectroscopy with High Reproducibility

A method is developed to synthesize surface-enhanced Raman scattering (SERS) materials capable of single-molecule detection, integrated with a microfluidic system. Using a focused laser, silver nanoparticle aggregates as SERS monitors are fabricated in a microfluidic channel through photochemical reduction. After washing out the monitor, the aggregates are irradiated again by the same laser. This key step leads to full reduction of the residual reactants, which generates numerous small silver nanoparticles on the former nanoaggregates. Consequently, the enhancement ability of the SERS monitor is greatly boosted due to the emergence of new "hot spots." At the same time, the influence of the notorious "memory effect" in microfluidics is substantially suppressed due to the depletion of surface residues. Taking these advantages, two-step photoreduced SERS materials are able to detect different types of molecules with the concentration down to 10^-13 m. Based on a well-accepted bianalyte approach, it is proved that the detection limit reaches the single-molecule level. From a practical point of view, the detection reproducibility at different probing concentrations is also investigated. It is found that the effective single-molecule SERS measurements can be raised up to ≈50%. This microfluidic SERS with high reproducibility and ultrasensitivity will find promising applications in on-chip single-molecule spectroscopy.

Nanoelectronic Discrimination of Nonmalignant and Malignant Cells Using Nanotube Field-Effect Transistors

Detection of malignant cells in tissue is a difficult hurdle in medical diagnostics and screening. Carbon nanotubes are extremely sensitive to their local environments, and nanotube-based field-effect transistors (NTFETs) provide a plethora of information regarding the mechanism of interaction with target analytes. Herein, we use a series of functionalized metal nanoparticle-decorated NTFET devices forming an array with multiple nonselective sensor units as the electronic "tongue", sensing all five basic tastes. By extraction of selected NTFET characteristics and using linear discriminant analysis, we have successfully detected and discriminated between malignant and nonmalignant tissues and cells. We also studied the sensing mechanism and what NTFET characteristics are responsible for the variation of response between cell types, allowing for the design of future studies such as detection of malignant cells in a biopsy or the effects of malignant cells on healthy tissue.

Materials for Microfluidic Immunoassays: A Review

Conventional immunoassays suffer from at least one of these following limitations: long processing time, high costs, poor user-friendliness, technical complexity, poor sensitivity and specificity. Microfluidics, a technology characterized by the engineered manipulation of fluids in channels with characteristic lengthscale of tens of micrometers, has shown considerable promise for improving immunoassays that could overcome these limitations in medical diagnostics and biology research. The combination of microfluidics and immunoassay can detect biomarkers with faster assay time, reduced volumes of reagents, lower power requirements, and higher levels of integration and automation compared to traditional approaches. This review focuses on the materials-related aspects of the recent advances in microfluidics-based immunoassays for point-of-care (POC) diagnostics of biomarkers. We compare the materials for microfluidic chips fabrication in five aspects: fabrication, integration, function, modification and cost, and describe their advantages and drawbacks. In addition, we review materials for modifying antibodies to improve the performance of the reaction of immunoassay. We also review the state of the art in microfluidic immunoassays POC platforms, from the laboratory to routine clinical practice, and also commercial products in the market. Finally, we discuss the current challenges and future developments in microfluidic immunoassays.

Plasmon-Based Colorimetric Nanosensors for Ultrasensitive Molecular Diagnostics

Colorimetric detection of target analytes with high specificity and sensitivity is of fundamental importance to clinical and personalized point-of-care diagnostics. Because of their extraordinary optical properties, plasmonic nanomaterials have been introduced into colorimetric sensing systems, which provide significantly improved sensitivity in various biosensing applications. Here we review the recent progress on these plasmonic nanoparticles-based colorimetric nanosensors for ultrasensitive molecular diagnostics. According to their different colorimetric signal generation mechanisms, these plasmonic nanosensors are classified into two categories: (1) interparticle distance-dependent colorimetric assay based on target-induced forming cross-linking assembly/aggregate of plasmonic nanoparticles; and (2) size/morphology-dependent colorimetric assay by target-controlled growth/etching of the plasmonic nanoparticles. The sensing fundamentals and cutting-edge applications will be provided for each of them, particularly focusing on signal generation and/or amplification mechanisms that realize ultrasensitive molecular detection. Finally, we also discuss the challenge and give our future perspective in this emerging field.

Aptabody-aptatope interactions in aptablotting assays

We demonstrate an aptablotting assay method that involves direct and indirect aptabody recognition. Nanoscale single-stranded DNA aptamers against GST and DIG-tags are utilized as aptabodies (GST-2 and DIG-1, respectively), and the GST-2 aptabody binding site, or aptatope, as predicted by a MOE-docking simulation of the protein-aptamer complex, shows the interaction of the GST-2 aptabody at the catalytically active region. The aptabody-aptatope interaction was evaluated by an in vitro enzyme inhibitory analysis. The binding capacity of the GST-2 aptabody was assessed by dot-blot, EMSA and SDS-PAGE/electroblot analyses, and the results showed that the aptabodies interact with both the native mono-/dimeric form and the denatured GST form on a membrane. The use of aptabodies can overcome the obstacles of current immunoblot assays, and these molecules are easily assessable via ELISA systems. Moreover, the hybridization of aptabodies and antibodies (hybrid-aptablotting) may have considerable impacts on the design of bioassay platforms.

Fe3O4 nanoparticles on graphene oxide sheets for isolation and ultrasensitive amperometric detection of cancer biomarker proteins

Ultrasensitive mediator-free electrochemical detection for biomarker proteins was achieved at low cost using a novel composite of Fe3O4 nanoparticles loaded onto graphene oxide (GO) nano-sheets (Fe3O4@GO). This paramagnetic Fe3O4@GO composite (1µm size range) was decorated with antibodies against prostate specific antigen (PSA) and prostate specific membrane antigen (PSMA), and then used to first capture these biomarkers and then deliver them to an 8-sensor detection chamber of a microfluidic immunoarray. Screen-printed carbon sensors coated with electrochemically reduced graphene oxide (ERGO) and a second set of antibodies selectively capture the biomarker-laden Fe3O4@GO particles, which subsequently catalyze hydrogen peroxide reduction to detect PSA and PSMA. Accuracy was confirmed by good correlation between patient serum assays and enzyme-linked immuno-sorbent assays (ELISA). Excellent detection limits (LOD) of 15 fg/mL for PSA and 4.8 fg/mL for PSMA were achieved in serum. The LOD for PSA was 1000-fold better than the only previous report of PSA detection using Fe3O4. Dynamic ranges were easily tunable for concentration ranges encountered in serum samples by adjusting the Fe3O4@GO Concentration. Reagent cost was only $0.85 for a single 2-protein assay.

Surface Modification of Gold Nanoparticles with Small Molecules for Biochemical Analysis

As one of the major tools for and by chemical science, biochemical analysis is becoming increasingly important in fields like clinical diagnosis, food safety, environmental monitoring, and the development of chemistry and biochemistry. The advancement of nanotechnology boosts the development of analytical chemistry, particularly the nanoparticle (NP)-based approaches for biochemical assays. Functional NPs can greatly improve the performance of biochemical analysis because they can accelerate signal transduction, enhance the signal intensity, and enable convenient signal readout due to their unique physical and chemical properties. Surface chemistry is a widely used tool to functionalize NPs, and the strategy for surface modification is of great significance to the application of NP-mediated biochemical assays. Surface chemistry not only affects the quality of NPs (stability, monodispersity, and biocompatibility) but also provides functional groups (-COO^-, -NH₃⁺, -CHO, and so on) or charges that can be exploited for bioconjugation or ligand exchange. Surface chemistry also dictates the sensitivity and specificity of the NP-mediated biochemical assays, since it is vital to the orientation, accessibility, and bioactivity of the functionalized ligands on the NPs. In this Account, we will focus on surface chemistry for functionalization of gold nanoparticles (AuNPs) with small organic molecules for biochemical analysis. Compared to other NPs, AuNPs have many merits including controllable synthesis, easy surface modification and high molar absorption coefficient, making them ideal probes for biochemical assays. Small-molecule functionalized AuNPs are widely employed to develop sensors for biochemical analysis, considering that small molecules, such as amino acids and sulfhydryl compounds, are more easily and controllably bioconjugated to the surface of AuNPs than biomacromolecules due to their less complex structure and steric hindrance. The orientation and accessibility of small molecules on AuNPs in most cases can be precisely controlled without compromising their bioactivity as well, thus ensuring the performance, such as the specificity and sensitivity, of AuNP-based biochemical assays. This Account reviews recent progress in the surface chemistry of functionalized AuNPs for biochemical assays. The surface chemistries mainly include click chemistry, ligand exchange reaction, and coordination-based recognition. These surface-modified AuNPs allow for assaying a range of important biochemical markers including metal ions, small biomolecules, enzymes, and antigens and antibodies. Applications of these systems range from environmental monitoring to medical diagnostics. This Account highlights the advantages and limitations (sensitivity, detection efficiency, and stability) that AuNP-mediated assays still have compared with conventional analytical methods. This Account also discusses the future research directions of surface-modified AuNP-mediated biochemical analysis. The main aim of this Account is to summarize the current surface modification strategies for AuNPs and further demonstrate how to make use of surface modification strategies to effectively improve the performance of AuNP-mediated analytical methods for a wide variety of applications relating to biochemical analysis.

Recent Advances in Synthesis and Biomedical Applications of Two-Dimensional Transition Metal Dichalcogenide Nanosheets

During recent decades, a giant leap in the development of nanotechnology has been witnessed. Numerous nanomaterials with different dimensions and unprecedented features have been developed and provided unimaginably wide scope to solve the challenging problems in biomedicine, such as cancer diagnosis and therapy. Recently, two-dimensional (2D) transition metal dichalcogenide (TMDC) nanosheets (NSs), including MoS₂ , WS₂ , and etc., have emerged as novel inorganic graphene analogues and attracted tremendous attention due to their unique structures and distinctive properties, and opened up great opportunities for biomedical applications, including ultrasensitive biosensing, biological imaging, drug delivery, cancer therapy, and antibacterial treatment. A comprehensive overview of different synthetic methods of ultrathin 2D TMDC NSs and their state-of-the-art biomedical applications, especially those that have appeared in the past few years, is presented. At the end of this review, the future opportunities and challenges for 2D TMDC NSs in biomedicine are also discussed.

A biosensor based on self-clickable AIEgen: a signal amplification strategy for ultrasensitive immunoassays

A signal amplified fluorogenic ELISA based on self-clickable fluorogen with aggregation-induced emission characteristics (AIEgen) as a substrate was developed for ultrasensitive immunoassay.

Slide cover glass immobilized liquid crystal microdroplets for sensitive detection of an IgG antigen

Slide cover glass immobilized AIgG conjugated LC microdroplets for optical detection of rabbit IgG antigen through interfacial antibody–antigen interactions.

Facile synthesis of thiazole-functionalized magnetic microspheres for highly specific separation of heme proteins

Thiazole-functionalized magnetic microspheres which exhibited high selectivity to capture hemoglobin with a binding capacity of 2.02 g g⁻¹ were successfully synthesized.

Redox active molybdophosphate produced by Cu3(PO4)2nanospheres for enhancing enzyme-free electrochemical immunoassay of C-reactive protein

Redox-active molybdophosphate produced by Cu₃(PO₄)₂nanospheres has been directly employed for signal amplification of an enzyme-free electrochemical immunosensor.

Understanding Toxicity of Nanomaterials in Biological Systems

Nanotechnology is a growing applied science having considerable global socioeconomic value. Nanoscale materials are casting their impact on almost all industries and all areas of society. A wide range of engineered nanoscale products has emerged with widespread applications in fields such as energy, medicine, electronics, plastics, energy and aerospace etc. While the market for nanotechnology products will have grown over one trillion US dollars by 2015, the presence of these material is likely to increase leading to increasing likelihood of exposure. The direct use of nanomaterials in humans for medical and cosmetic purposes dictates vigorous safety assessment of toxicity. Therefore this book chapter provides the detailed toxicity assessment of various types of nanomaterials.

An enzyme-mediated competitive colorimetric sensor based on Au@Ag bimetallic nanoparticles for highly sensitive detection of disease biomarkers

An Au@Ag nanosensor for highly sensitive, qualitative and quantitative immunoassays.

Multiplex Immunosensor Arrays for Electrochemical Detection of Cancer Biomarker Proteins

Measuring panels of protein biomarkers offer a new personalized approach to early cancer detection, disease monitoring and patients' response to therapy. Multiplex electrochemical methods are uniquely positioned to provide faster, more sensitive, point of care (POC) devices to detect protein biomarkers for clinical diagnosis. Nanomaterials-based electrochemical methods offer sensitivity needed for early cancer detection. This review discusses recent advances in multiplex electrochemical immunosensors for cancer diagnostics and disease monitoring. Different electrochemical strategies including enzyme-based immunoarrays, nanoparticle-based immunoarrays and electrochemiluminescence methods are discussed. Many of these methods have been integrated into microfluidic systems, but measurement of more than 2-4 protein markers in a small single serum sample is still a challenge. For POC applications, a simple, low cost method is required. Major challenges in multiplexed microfluidic immunoassays are reagent additions and washing steps that require creative engineering solutions. 3-D printed microfluidics and paper-based microfluidic devices are also explored.

Plasmonically Engineered Nanoprobes for Biomedical Applications

The localized surface plasmon resonance of metal nanoparticles is the collective oscillation of electrons on particle surface, induced by incident light, and is a particle composition-, morphology-, and coupling-dependent property. Plasmonic engineering deals with highly precise formation of the targeted nanostructures with targeted plasmonic properties (e.g., electromagnetic field distribution and enhancement) via controlled synthetic, assembling, and atomic/molecular tuning strategies. These plasmonically engineered nanoprobes (PENs) have a variety of unique and beneficial physical, chemical, and biological properties, including optical signal enhancement, catalytic, and local temperature-tuning photothermal properties. In particular, for biomedical applications, there are many useful properties from PENs including LSPR-based sensing, surface-enhanced Raman scattering, metal-enhanced fluorescence, dark-field light-scattering, metal-mediated fluorescence resonance energy transfer, photothermal effect, photodynamic effect, photoacoustic effect, and plasmon-induced circular dichroism. These properties can be utilized for the development of new biotechnologies and biosensing, bioimaging, therapeutic, and theranostic applications in medicine. This Perspective introduces the concept of plasmonic engineering in designing and synthesizing PENs for biomedical applications, gives recent examples of biomedically functional PENs, and discusses the issues and future prospects of PENs for practical applications in bioscience, biotechnology, and medicine.

Design of dual working electrodes for concentration process in metalloimmunoassay

Electrochemical immunosensing, particularly through a metalloimmunoassay, is a promising approach for development of point-of-care (POC) diagnostics devices. This study investigated the structure of dual working electrodes (W1 and W2), used in a silver nanoparticles-labeled sandwich-type immunoassay and silver concentration process, paying special attention to the position of W1 relative to W2. The new structures of the dual working electrodes were fabricated for efficient silver concentration and evaluated experimentally, which showed that the duration of prereduction before current measurement decreased from 480 s to 300 s by transforming the position of W1 from 1 line to 2 lines or 6 parts. The experimental results were also compared with numerical simulations based on three-dimensional diffusion, and the prereduction step almost followed the three-dimensional diffusion equation. Using numerical simulations, the ideal structures of dual working electrodes were designed based on relationships between the structures and duration of prereduction or the LOD. In the case of 36 lines at an area ratio of W1 to W1 + W2 of 1 to 10, the prereduction duration decreased to 96 s. The dual working electrodes designed in this study promise to shorten the total analysis time and lower the LOD for POC diagnostics.

High-Throughput Electrochemical Microfluidic Immunoarray for Multiplexed Detection of Cancer Biomarker Proteins

Microchip-based microfluidic electrochemical arrays hold great promise for fast, high-throughput multiplexed detection of cancer biomarker proteins at low cost per assay using relatively simple instrumentation. Here we describe an inexpensive high-throughput electrochemical array featuring 32 individually addressable microelectrodes that is further multiplexed with an 8-port manifold to provide 256 sensors. The gold electrode arrays were fabricated by wet-etching commercial gold compact discs (CD-R) followed by patterned insulation. A print-and-peel method was used to create sub-microliter hydrophobic wells surrounding each sensor to eliminate cross contamination during immobilization of capture antibodies. High-throughput analyses were realized using eight 32-sensor immunoarrays connected to the miniaturized 8-port manifold, allowing 256 measurements in <1 h. This system was used to determine prostate cancer biomarker proteins prostate specific antigen (PSA), prostate specific membrane antigen (PSMA), interleukin-6 (IL-6), and platelet factor-4 (PF-4) in serum. Clinically relevant detection limits (0.05 to 2 pg mL^-1) and 5-decade dynamic ranges (sub pg mL^-1 to well above ng mL^-1) were achieved for these proteins utilizing precapture of analyte proteins on magnetic nanoparticles decorated with enzyme labels and antibodies.

Targeted multimodal theranostics via biorecognition controlled aggregation of metallic nanoparticle composites

We have developed a theranostic nanocomposite of metallic nanoparticles that uses two distinct fluorescence mechanisms: Förster Resonance Energy Transfer (FRET) and Metal-Enhanced Fluorescence (MEF) controlled by ligand-receptor interaction. Supramolecular assembly of the fluorophore-labeled glycoligands to cyclodextrin-capped gold nanoparticles produces a nanocomposite with a quenched fluorescence due to FRET from the fluorophore to the proximal particle. Subsequently, interaction with a selective protein receptor leads to an aggregation of the composite, reactivating the fluorescence by MEF from the distal metallic particles to fluorophores encapsulated in the aggregates. The aggregation also causes a red-shift in absorbance of the composite, thereby enhancing the production of reactive oxygen species (ROS) on red-light irradiation. Our nanocomposite has proven suitable for targeted cancer cell imaging as well as multimode therapy using both the photodynamic and drug delivery properties of the composite.

Ultrasensitive Detection of Low-Abundance Protein Biomarkers by Mass Spectrometry Signal Amplification Assay

A mass spectrometry signal amplification method is developed for the ultrasensitive and selective detection of low-abundance protein biomarkers by utilizing tag molecules on gold nanoparticles (AuNPs). EpCAM and thrombin as model targets are captured by specific aptamers immobilized on the AuNPs. With laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS), the mass tag molecules are detected to represent the protein biomarkers. Benefiting from the MS signal amplification, the assay can achieve a limit of detection of 100 aM. The method is further applied to detect thrombin in fetal bovine serum and EpCAM in cell lysates to demonstrate its selectivity and feasibility in complex biological samples. With the high sensitivity and specificity, the protocol shows great promise for providing a new route to single-cell analysis and early disease diagnosis.

Quantifying Nanomolar Protein Concentrations Using Designed DNA Carriers and Solid-State Nanopores

Designed "DNA carriers" have been proposed as a new method for nanopore based specific protein detection. In this system, target protein molecules bind to a long DNA strand at a defined position creating a second level transient current drop against the background DNA translocation. Here, we demonstrate the ability of this system to quantify protein concentrations in the nanomolar range. After incubation with target protein at different concentrations, the fraction of DNA translocations showing a secondary current spike allows for the quantification of the corresponding protein concentration. For our proof-of-principle experiments we use two standard binding systems, biotin-streptavidin and digoxigenin-antidigoxigenin, that allow for measurements of the concentration down to the low nanomolar range. The results demonstrate the potential for a novel quantitative and specific protein detection scheme using the DNA carrier method.

Metal nanoclusters: novel probes for diagnostic and therapeutic applications

Metal nanoclusters, composed of several to a few hundred metal atoms, have received worldwide attention due to their extraordinary physical and chemical characteristics. Recently, great efforts have been devoted to the exploration of the potential diagnostic and therapeutic applications of metal nanoclusters. Here we focus on the recent advances and new horizons in this area, and introduce the rising progress on the use of metal nanoclusters for biological analysis, biological imaging, therapeutic applications, DNA assembly and logic gate construction, enzyme mimic catalysis, as well as thermometers and pH meters. Furthermore, the future challenges in the construction of biofunctional metal nanoclusters for diagnostic and therapeutic applications are also discussed. We expect that the rapidly growing interest in metal nanocluster-based theranostic applications will certainly not only fuel the excitement and stimulate research in this highly active field, but also inspire broader concerns across various disciplines.

A portable smart phone-based plasmonic nanosensor readout platform that measures transmitted light intensities of nanosubstrates using an ambient light sensor

Plasmonic nanosensors may be used as tools for diagnostic testing in the field of medicine. However, quantification of plasmonic nanosensors often requires complex and bulky readout instruments. Here, we report the development of a portable smart phone-based plasmonic nanosensor readout platform (PNRP) for accurate quantification of plasmonic nanosensors. This device operates by transmitting excitation light from a LED through a nanosubstrate and measuring the intensity of the transmitted light using the ambient light sensor of a smart phone. The device is a cylinder with a diameter of 14 mm, a length of 38 mm, and a gross weight of 3.5 g. We demonstrated the utility of this smart phone-based PNRP by measuring two well-established plasmonic nanosensors with this system. In the first experiment, the device measured the morphology changes of triangular silver nanoprisms (AgNPRs) in an immunoassay for the detection of carcinoembryonic antigen (CEA). In the second experiment, the device measured the aggregation of gold nanoparticles (AuNPs) in an aptamer-based assay for the detection of adenosine triphosphate (ATP). The results from the smart phone-based PNRP were consistent with those from commercial spectrophotometers, demonstrating that the smart phone-based PNRP enables accurate quantification of plasmonic nanosensors.