Electrochemical coding for multiplexed immunoassays of proteins

An enzyme-linked immuno-mass spectrometric assay with the substrate adenosine monophosphate

An enzyme-linked immuno-mass spectrometric assay (ELIMSA) with the specific detection probe streptavidin conjugated to alkaline phosphatase catalyzed the production of adenosine from the substrate adenosine monophosphate (AMP) for sensitive quantification of prostate-specific antigen (PSA) by mass spectrometry. Adenosine ionized efficiently and was measured to the femtomole range by dilution and direct analysis with micro-liquid chromatography, electrospray ionization, and mass spectrometry (LC-ESI-MS). The LC-ESI-MS assay for adenosine production was shown to be linear and accurate using internal (13)C(15)N adenosine isotope dilution, internal (13)C(15)N adenosine one-point calibration, and external adenosine standard curves with close agreement. The detection limits of LC-ESI-MS for alkaline phosphatase-streptavidin (AP-SA, ∼190,000 Da) was tested by injecting 0.1 μl of a 1 pg/ml solution, i.e., 100 attograms or 526 yoctomole (5.26E-22) of the alkaline-phosphatase labeled probe on column (about 315 AP-SA molecules). The ELIMSA for PSA was linear and showed strong signals across the picogram per milliliter range and could robustly detect PSA from all of the prostatectomy patients and all of the female plasma samples that ranged as low as 70 pg/ml with strong signals well separated from the background and well within the limit of quantification of the AP-SA probe. The results of the ELIMSA assay for PSA are normal and homogenous when independently replicated with a fresh standard over multiple days, and intra and inter diem assay variation was less than 10 % of the mean. In a blind comparison, ELIMSA showed excellent agreement with, but was more sensitive than, the present gold standard commercial fluorescent ELISA, or ECL-based detection, of PSA from normal and prostatectomy samples, respectively.

Pyridoxamine-5-phosphate enzyme-linked immune mass spectrometric assay substrate for linear absolute quantification of alkaline phosphatase to the yoctomole range applied to prostate specific antigen

There is a need to measure proteins that are present in concentrations below the detection limits of existing colorimetric approaches with enzyme-linked immunoabsorbent assays (ELISA). The powerful enzyme alkaline phosphatase conjugated to the highly specific bacterial protein streptavidin binds to biotinylated macromolecules like proteins, antibodies, or other ligands and receptors with a high affinity. The binding of the biotinylated detection antibody, with resulting amplification of the signal by the catalytic production of reporter molecules, is key to the sensitivity of ELISA. The specificity and amplification of the signal by the enzyme alkaline phosphatase in ELISA together with the sensitivity of liquid chromatography electrospray ionization and mass spectrometry (LC-ESI-MS) to detect femtomole to picomole amounts of reporter molecules results in an ultrasensitive enzyme-linked immune mass spectrometric assay (ELIMSA). The novel ELIMSA substrate pyridoxamine-5-phosphate (PA5P) is cleaved by the enzyme alkaline phosphatase to yield the basic and hydrophilic product pyridoxamine (PA) that elutes rapidly with symmetrical peaks and a flat baseline. Pyridoxamine (PA) and (13)C PA were both observed to show a linear relationship between log ion intensity and quantity from picomole to femtomole amounts by liquid chromatography-electrospray ionization and mass spectrometry. Four independent methods, (i) internal (13)C isotope PA dilution curves, (ii) internal (13)C isotope one-point calibration, (iii) external PA standard curve, and (iv) external (13)C PA standard curve, all agreed within 1 digit in the same order of magnitude on the linear quantification of PA. Hence, a mass spectrometer can be used to robustly detect 526 ymol of the alkaline phosphatase streptavidin probe and accurately quantify zeptomole amounts of PSA against log linear absolute standard by micro electrospray on a simple ion trap.

Proximity hybridization regulated DNA biogate for sensitive electrochemical immunoassay

An electrochemical DNA biogate was designed for highly sensitive homogeneous electrochemical immunoassay by combining target-induced proximity hybridization with a mesoporous silica nanoprobe (MSN). The electroactive methylene blue (MB) was sealed in the inner pores of MSN with single-stranded DNA. In the presence of target protein and two DNA-labeled antibodies, the formed proximate complex could hybridize with the DNA strand to form a rigid double-stranded structure and thus open the biogate, which led to the release of MB entrapped in the MSN. The target protein-dependent amount of released MB could be conveniently monitored with a screen-printed carbon electrode. Moreover, the detachment process of MB could be further amplified with an in situ enzymatic recycling binding of the proximate complex with the single-stranded DNA. Using prostate-specific antigen as a model target, the proposed assay showed a wide detection range from 0.002 to 100 ng mL(-1) with a detection limit of 1.3 pg mL(-1). This strategy was simple and universal for various analytes with different affinity ligands. This method possessed great potential for convenient point-of-care testing and commercial application.

Bioinspired nanoscale materials for biomedical and energy applications

The demand for green, affordable and environmentally sustainable materials has encouraged scientists in different fields to draw inspiration from nature in developing materials with unique properties such as miniaturization, hierarchical organization and adaptability. Together with the exceptional properties of nanomaterials, over the past century, the field of bioinspired nanomaterials has taken huge leaps. While on the one hand, the sophistication of hierarchical structures endows biological systems with multi-functionality, the synthetic control on the creation of nanomaterials enables the design of materials with specific functionalities. The aim of this review is to provide a comprehensive, up-to-date overview of the field of bioinspired nanomaterials, which we have broadly categorized into biotemplates and biomimics. We discuss the application of bioinspired nanomaterials as biotemplates in catalysis, nanomedicine, immunoassays and in energy, drawing attention to novel materials such as protein cages. Furthermore, the applications of bioinspired materials in tissue engineering and biomineralization are also discussed.

High throughput immunosenor based on multi-label strategy and a novel array electrode

Accurate prediction of a particular cancer can be achieved by measuring multiplex biomarkers. Traditional methods for multi-biomarkers detection are either multi-spots assay with chip or multi-label assay with one detection spot. However, the detection throughput of these two approaches is limited by the substrate area and the numbers of available label respectively. To solve this problem, in the present study, an immunoassay was firstly prepared by combining multi-label strategy and multi-spot assay with a novel array electrode for simultaneous detection of six biomarkers for hepatocellular carcinoma (HCC). The detection throughput of the proposed method was doubled in comparison with traditional multi-spots assay (one target protein was detected on each analytic spot), which could greatly enhance the sensitivity and specificity of HCC diagnosis. This detection model may serve as the starting point for high throughput of multianalyte assay.

Nanomaterials and biomaterials in electrochemical arrays for protein detection

Nanomaterials and biomaterials are important components of new electrochemical arrays designed for sensitive detection of proteins in biological fluids. Such multiplexed protein arrays are predicted to have an important future in personalized medical diagnostics, especially for cancer and heart disease. Sandwich immunoassays for proteins benefit greatly in sensitivity from the use of nanostructured sensor surfaces and multilabeled detection strategies involving nano- or microparticles. In these assays, capture agents such as antibodies or aptamers are attached to sensor surfaces in the array. Target proteins with large binding constants for the affinity agents are captured from liquid samples with high efficiency, either on the sensors or on magnetic bioconjugate particles decorated with many copies of labels and antibodies. After target proteins are captured on the sensor surfaces, the labels are detected by electrochemical techniques. This feature article begins with an overview of the recent history of nanoparticles in electrochemical protein sensors, then moves on to specific examples from our own laboratories. We discuss fabrication of nanostructured sensors and arrays with the aim of multiplexed detection as well as reusability. Following this, we describe systems that integrate particle-based protein sensing with microfluidics for multiplexed protein detection. We end with predictions on the diagnostic future of protein detection.

A dual amplification strategy for ultrasensitive electrochemiluminescence immunoassay based on a Pt nanoparticles dotted graphene–carbon nanotubes composite and carbon dots functionalized mesoporous Pt/Fe

A facile and sensitive ECL immunosensor has been designed using Pt/Gr–CNTs as a platform and Pt/Fe@CDs as bionanolabels.

Multiplexed electrochemical protein detection and translation to personalized cancer diagnostics

Measuring diagnostic panels of multiple proteins promises a new, personalized approach to early detection and therapy of diseases like cancer. Levels of biomarker proteins in patient serum can provide a continually updated record of disease status. Research in electrochemical detection of proteins has produced exquisitely sensitive approaches. Most utilize ELISA-like sandwich immunoassays incorporating various aspects of nanotechnology. Several of these ultrasensitive methodologies have been extended to microfluidic multiplexed protein detection, but engineered solutions are needed to measure more proteins in a single device from a small patient sample such as a drop of blood or tissue lysate. To achieve clinical or point-of-care (POC) use, simplicity and low cost are essential. In multiplexed microfluidic immunoassays, required reagent additions and washing steps pose a significant problem calling for creative engineering. A grand challenge is to develop a general cancer screening device to accurately measure 50-100 proteins in a simple, cost-effective fashion. This will require creative solutions to simplified reagent addition and multiplexing.

Supersandwich cytosensor for selective and ultrasensitive detection of cancer cells using aptamer-DNA concatamer-quantum dots probes

In this work, a signal amplification supersandwich strategy was developed for highly selective and sensitive detection of cancer cells using aptamer-DNA concatamer-quantum dots (QDs) probes. First of all, electrode materials denoted as MWCNTs@PDA@AuNPs were fabricated by multiwall carbon nanotubes (MWCNTs), gold nanoparticles (AuNPs), and polydopamine (PDA) using a layer-by-layer technique. Then, the prepared bases as matrices were applied to bind concanavalin A (Con A), resulting in high stability, bioactivity, and capability for cell capture. Meanwhile, aptamer-DNA concatamer-QDs were designed via DNA hybridization followed by covalent assembling, which incorporated the specific recognition of the aptamer with the signal amplification of the DNA concatamer and QDs. With aptamer-DNA concatamer-QDs as recognizing probes, the model cancer cells (CCRF-CEM cells) were detected using a MWCNTs@PDA@AuNPs modified electrode with trapped Con A by means of fluorescence and electrochemical methods. The proposed supersandwich cytosensor showed high sensitivity with the detection limit of 50 cells mL(-1). More importantly, it could distinguish cancer cells from normal cells, which indicated the promising applications of our method in clinical diagnosis and treatment of cancers.

Development of a Coulombimetric immunosensor based on specific antibodies labeled with CdS nanoparticles for sulfonamide antibiotic residues analysis and its application to honey samples

A new electrochemical immunosensor has been developed to detect sulfonamide antibiotic residues in food samples. The immunosensor presented uses immunoreagents specifically developed for the broad recognition of the sulfonamide antibiotic family, a graphite composite electrode (GEC), biofunctionalized magnetic μ-particles and electrochemical nanoprobes prepared by labeling the specific antibodies with CdS nanoparticles (CdSNP). After the immunochemical reaction, the CdSNP are dissolved and the metal ions released are reduced at the electrode and read as in the form of current or charge signal, by the well-known anodic stripping technique. Due to the amplification effect on the amperometric/coulombimetric signal produced by the CdSNP, a high detectability can be reached. Thus, sulfapyridine (SPY), one of the most widely used sulfonamide congeners, can be detected in buffer with an IC50current of 0.20±0.25μgL(-1). The immunosensor has been applied to the analysis of residues of this antibiotic in honey samples. Due to the reported formation of sulfonamide-sugar conjugates in this type of matrix, honey samples are first hydrolyzed in acidic media. The use of magnetic particles minimizes the matrix effect allowing to reach a detectability (LOD, limit of detection) of 0.11μgkg(-1) (current measurements), far below the limits established in some countries for these types of residues in honey samples. Due to the use of magnetic racks, multiple samples can be run simultaneously. The whole analysis process can be performed in around 22min.

Functionalized CdSe/ZnS QDs for the detection of nitroaromatic or RDX explosives

Chemically modified CdSe/ZnS quantum dots (QDs) are used as fluorescent probes for the analysis of explosives, and specifically, the detection of trinitrotoluene (TNT) or trinitrotriazine (RDX). The QDs are functionalized with electron-donating ligands that bind nitro-containing explosives, exhibiting electron-acceptor properties, to the QD surface, via supramolecular donor-acceptor interactions leading to the quenching of the luminescence of the QDs.

Development of copper based drugs, radiopharmaceuticals and medical materials

Copper is one of the most interesting elements for various biomedical applications. Copper compounds show vast array of biological actions, including anti-inflammatory, anti-proliferative, biocidal and other. It also offers a selection of radioisotopes, suitable for nuclear imaging and radiotherapy. Quick progress in nanotechnology opened new possibilities for design of copper based drugs and medical materials. To date, copper has not found many uses in medicine, but number of ongoing research, as well as preclinical and clinical studies, will most likely lead to many novel applications of copper in the near future.

Battery-triggered microfluidic paper-based multiplex electrochemiluminescence immunodevice based on potential-resolution strategy

A potential-resolution strategy for multiplex electrochemiluminescence (ECL) immunoassay on a microfluidic paper-based analytical device (μ-PAD) was demonstrated for the first time, using tris-(bipyridine)-ruthenium(ii) (Ru(bpy)(3)(2+)) and carbon nanodots (CNDs) as the ECL labels for high-throughput ECL immunoassay on μ-PAD. Based on this strategy, simultaneous detection of four tumor markers using only two screen-printed carbon working electrodes (one electrode for two analytes) on an immunodevice consisting of a piece of patterned paper (denoted as μ-PECLI in this work) was realized, which is a simplification of the configuration of traditional μ-PADs. As a further development of μ-PECLI in low-cost and disposable applications, battery-triggered (constant-potential) ECL detection on μ-PECLI was developed, allowing the traditional electrochemical workstation to be abandoned. In addition, to exactly control the output voltage of the battery, a low-cost and simple voltage-controller was designed and fabricated for the first time. The battery-triggered ECL immunoassay principle on μ-PECLI is explained. We found that the simultaneous determination of two analytes in one paper working zone could be achieved by controlling the operational constant-potential (+1.2 V for Ru(bpy)(3)(2+) labels and -1.2 V for CNDs labels (vs. Ag/AgCl auxiliary electrode)) by simply reversing the connection mode. Finally, four tumor markers in human serum samples from the Tumor Hospital were assayed and the results obtained were in acceptable agreement with the reference values from parallel single-analyte test. This battery-triggered μ-PECLI provides a new strategy for high-throughput, low-cost, sensitive, automated and simultaneous multiplex immunoassay and point-of-care diagnosis.

Applications of quantum dots as probes in immunosensing of small-sized analytes

Quantum dots (QDs) are semiconductor nanoparticles with very interesting optical properties, like high quantum yield or narrow and size-tuneable fluorescence spectra. Current applications of QDs are widespread, their use as fluorescence labels in bioassays being one of the most promising. These nanoparticles are usually conjugated to highly specific biomolecules like antibodies, oligonucleotides, enzymes or aptamers to improve assay selectivity. In this review, QD surface passivation, conjugation to biomolecules, and purification strategies are discussed with special emphasis to the development of QD-based immunoassays for the detection of low molecular weight compounds given the relevance of this sort of analytes in health, food safety, pharmaceutical, or environmental monitoring areas. The aim of this review is to summarise the main achievements attained so far and to initialise researchers in the field of antibody-based assays employing QDs as labels, such as fluorescence-linked immunosorbent assay (FLISA), fluorescence (or Förster) resonance energy transfer (FRET), immunochromatographic methods, and immunosensors.

3D origami-based multifunction-integrated immunodevice: low-cost and multiplexed sandwich chemiluminescence immunoassay on microfluidic paper-based analytical device

A novel 3D microfluidic paper-based immunodevice, integrated with blood plasma separation from whole blood samples, automation of rinse steps, and multiplexed CL detections, was developed for the first time based on the principle of origami (denoted as origami-based device). This 3D origami-based device, comprised of one test pad surrounded by four folding tabs, could be patterned and fabricated by wax-printing on paper in bulk. In this work, a sandwich-type chemiluminescence (CL) immunoassay was introduced into this 3D origami-based immunodevice, which could separate the operational procedures into several steps including (i) folding pads above/below and (ii) addition of reagent/buffer under a specific sequence. The CL behavior, blood plasma separation, washing protocol, and incubation time were investigated in this work. The developed 3D origami-based CL immunodevice, combined with a typical luminuol-H(2)O(2) CL system and catalyzed by Ag nanoparticles, showed excellent analytical performance for the simultaneous detection of four tumor markers. The whole blood samples were assayed and the results obtained were in agreement with the reference values from the parallel single-analyte test. This paper-based microfluidic origami CL detection system provides a new strategy for a low-cost, sensitive, simultaneous multiplex immunoassay and point-of-care diagnostics.

Paramagnetic particles coupled with an automated flow injection analysis as a tool for influenza viral protein detection

Currently, the influenza virus infects millions of individuals every year. Since the influenza virus represents one of the greatest threats, it is necessary to develop a diagnostic technique that can quickly, inexpensively, and accurately detect the virus to effectively treat and control seasonal and pandemic strains. This study presents an alternative to current detection methods. The flow-injection analysis-based biosensor, which can rapidly and economically analyze a wide panel of influenza virus strains by using paramagnetic particles modified with glycan, can selectively bind to specific viral A/H5N1/Vietnam/1203/2004 protein-labeled quantum dots. Optimized detection of cadmium sulfide quantum dots (CdS QDs)-protein complexes connected to paramagnetic microbeads was performed using differential pulse voltammetry on the surface of a hanging mercury drop electrode (HMDE) and/or glassy carbon electrode (GCE). Detection limit (3 S/N) estimations based on cadmium(II) ions quantification were 0.1 μg/mL or 10 μg/mL viral protein at HMDE or GCE, respectively. Viral protein detection was directly determined using differential pulse voltammetry Brdicka reaction. The limit detection (3 S/N) of viral protein was estimated as 0.1 μg/mL. Streptavidin-modified paramagnetic particles were mixed with biotinylated selective glycan to modify their surfaces. Under optimized conditions (250 μg/mL of glycan, 30-min long interaction with viral protein, 25°C and 400 rpm), the viral protein labeled with quantum dots was selectively isolated and its cadmium(II) content was determined. Cadmium was present in detectable amounts of 10 ng per mg of protein. Using this method, submicrogram concentrations of viral proteins can be identified.

Ultrasensitive multianalyte electrochemical immunoassay based on metal ion functionalized titanium phosphate nanospheres

A novel multianalyte electrochemical immunoassay was developed for ultrasensitive detection of human cardiopathy biomarkers cardiac troponin I (cTnI) and human heart-type fatty-acid-binding protein (FABP) using metal ion functionalized titanium phosphate nanospheres (TiP-metal ion) as labels. The metal ions could be detected directly through square wave voltammetry (SWV) without metal preconcentration, and the distinct voltammetric peaks had a close relationship with each sandwich-type immunoreaction. The position and size of the peaks reflected the identity and level of the corresponding antigen. The large amount of metal ions loading on the TiP nanospheres greatly amplified the detection signals, and the good biocompatibility of graphene nanoribbons (GONRs) retained good stability for the sandwich-type immunoassay. The proposed immunoassay exhibited high sensitivity and selectivity for the detection of cTnI and FABP. The linear relationships between electrochemical signals and the concentrations of cTnI and FABP were obtained in the range of 0.05 pg/mL-50 ng/mL and 0.05 pg/mL-50 ng/mL, respectively. The detection limits of cTnI and HIgG were 1 and 3 fg/mL (S/N = 3), respectively. Moreover, the immunoassay accurately detected the concentrations of cTnI and FABP in human serum samples, which were demonstrated to have excellent correlations with the standard enzyme linked immunosorbent assay (ELISA) method. The results suggested that the electrochemical immunoassay would be promising in the point-of-care diagnostics application of clinical screening of acute myocardial infarction (AMI) biomarkers.

Electrochemical immunosensors for detection of cancer protein biomarkers

Bioanalytical methods have experienced unprecedented growth in recent years, driven in large part by the need for faster, more sensitive, more portable ("point of care") systems to detect protein biomarkers for clinical diagnosis. Electrochemical detection strategies, used in conjunction with immunosensors, offer advantages because they are fast, simple, and low cost. Recent developments in electrochemical immunosensors have significantly improved the sensitivity needed to detect low concentrations of biomarkers present in early stages of cancer. Moreover, the coupling of electrochemical devices with nanomaterials, such as gold nanoparticles, carbon nanotubes, magnetic particles, and quantum dots, offers multiplexing capability for simultaneous measurements of multiple cancer biomarkers. This review will discuss recent advances in the development of electrochemical immunosensors for the next generation of cancer diagnostics, with an emphasis on opportunities for further improvement in cancer diagnostics and treatment monitoring. Details will be given for strategies to increase sensitivity through multilabel amplification, coupled with high densities of capture molecules on sensor surfaces. Such sensors are capable of detecting a wide range of protein quantities, from nanogram to femtogram (depending on the protein biomarkers of interest), in a single sample.

A sulfhydryl-reactive ruthenium (II) complex and its conjugation to protein G as a universal reagent for fluorescent immunoassays

To develop a fluorescent ruthenium complex for biosensing, we synthesized a novel sulfhydryl-reactive compound, 4-bromophenanthroline bis-2,2′-dipyridine Ruthenium bis (hexafluorophosphate). The synthesized Ru(II) complex was crosslinked with thiol-modified protein G to form a universal reagent for fluorescent immunoassays. The resulting Ru(II)-protein G conjugates were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The emission peak wavelength of the Ru(II)-protein G conjugate was 602 nm at the excitation of 452 nm which is similar to the spectra of the Ru(II) complex, indicating that Ru(II)-protein G conjugates still remain the same fluorescence after conjugation. To test the usefulness of the conjugate for biosensing, immunoglobulin G (IgG) binding assay was conducted. The result showed that Ru(II)-protein G conjugates were capable of binding IgG and the more cross-linkers to modify protein G, the higher conjugation efficiency. To demonstrate the feasibility of Ru(II)-protein G conjugates for fluorescent immunoassays, the detection of recombinant histidine-tagged protein using the conjugates and anti-histidine antibody was developed. The results showed that the histidine-tagged protein was successfully detected with dose-response, indicating that Ru(II)-protein G conjugate is a useful universal fluorescent reagent for quantitative immunoassays.

On-chip electrochemical detection of CdS quantum dots using normal and multiple recycling flow through modes

A flexible hybrid polydimethylsiloxane (PDMS)-polycarbonate (PC) microfluidic chip with integrated screen printed electrodes (SPE) was fabricated and applied for electrochemical quantum dots (QDs) detection. The developed device combines the advantages of flexible microfluidic chips, such as their low cost, the possibility to be disposable and amenable to mass production, with the advantages of electrochemistry for its facility of integration and the possibility to miniaturize the analytical device. Due to the interest in biosensing applications in general and particularly the great demand for labelling alternatives in affinity biosensors, the electrochemistry of cadmium sulfide quantum dots (CdS QDs) is evaluated. Square wave anodic stripping voltammetry (SWASV) is the technique used due to its sensitivity and low detection limits that can be achieved. The electrochemical as well as the microfluidic parameters of the developed system are optimized. The detection of CdS QDs in the range between 50 to 8000 ng mL(-1) with a sensitivity of 0.0009 μA/(ng mL(-1)) has been achieved. In addition to the single in-chip flow through measurements, the design of a recirculation system with the aim of achieving lower detection limits using reduced volumes (25 μL) of sample was proposed as a proof-of-concept.

Towards in vitro molecular diagnostics using nanostructures

Nanostructures appear to be promising for a number of applications in molecular diagnostics, mainly due to the increased surface-to-volume ratio they can offer, the very low limit of detection achievable, and the possibility to fabricate point-of-care diagnostic devices. In this paper, we review examples of the use of nanostructures as diagnostic tools that bring in marked improvements over prevalent classical assays. The focus is laid on the various sensing paradigms that possess the potential or have demonstrated the capability to replace or augment current analytical strategies. We start with a brief introduction of the various types of nanostructures and their physical properties that determine the transduction principle. This is followed by a concise collection of various functionalization protocols used to immobilize biomolecules on the nanostructure surface. The sensing paradigms are discussed in two contexts: the nanostructure acting as a label for detection, or the nanostructure acting as a support upon which the molecular recognition events take place. In order to be successful in the field of molecular diagnostics, it is important that the nanoanalytical tools be evaluated in the appropriate biological environment. The final section of the review compiles such examples, where the nanostructure-based diagnostic tools have been tested on realistic samples such as serum, demonstrating their analytical power even in the presence of complex matrix effects. The ability of nanodiagnostic tools to detect ultralow concentrations of one or more analytes coupled with portability and the use of low sample volumes is expected to have a broad impact in the field of molecular diagnostics.

Nanomaterials-based electrochemical immunosensors for proteins

For this special issue on 90 years of polarography, the following personal account describes how my early research in electrochemistry and polarography in the laboratory of Prof. Petr Zuman led to a major research effort in the determination of proteins for cancer detection and monitoring. It reviews the very recent history of nanoparticle labels and multiplexed detection in protein immunosensors. It then describes our journey of discovery that has led to ultrasensitive protein immunosensors achieved by combining nanostructured electrodes with particles labeled with up to ½ million enzymes that can detect down to as little as 1 fg mL(-1) protein in diluted serum. Our most mature multiple protein detection system is a microfluidic device with eight sensors coated with 5-nm gold nanoparticles that uses off-line protein detection with heavily labeled magnetic particles. This approach has led to reliable sub pg mL(-1) detection limits for multiple proteins, provides excellent correlation with referee ELISA methods, and is currently being used for validation of panels of biomarkers for oral and prostate cancer. The article ends with a section on future perspectives.