One-Step Homogeneous DNA Assay with Single-Nanoparticle Detection

Amplification-Free Strategy for miRNA Quantification in Human Serum Using Single Particle ICP-MS and Gold Nanoparticles as Labels

MicroRNAs (miRNAs), which are short single-stranded RNA sequences between 18 and 24 nucleotides, are known to play a crucial role in gene expression. Changes in their expression are not only involved in many diseases but also as a response to physiological changes, such as physical exercise. In this work, a new analytical strategy for the sensitive and specific analysis of miRNA sequences in human plasma is presented. The developed strategy does not depend on any nucleic acid amplification process and can be obtained in direct correlation to the number of events obtained by using single-particle ICP-MS measurements. The high selectivity of the assay (up to single nucleotide polymorphisms) can be achieved by a double hybridization process of the target miRNA with a complementary capture oligonucleotide that is conjugated to a magnetic microparticle and simultaneously with a complementary reporter oligonucleotide conjugated to a gold nanoparticle. Thanks to the novel approach followed in this method, the stoichiometry of the oligonucleotide-nanoparticle conjugates does not need to be addressed for the quantification of the target miRNA, which also represents a big advantage over other similar methods. The optimized method is applied to the determination of a miRNA as a biomarker of physical exercise in non-spiked human serum samples, and the results are validated against rt-qPCR. The achieved sensitivity permits the direct differentiation among sedentary and sportive subjects. This general platform can be easily applied to any other sequence by only modifying the capture and reporter oligonucleotides, paving the way for multiple clinically interesting applications.

Highly sensitive detection of aflatoxin B1 byCRISPR/Cas12a-assisted single nanoparticle counting

CRISPR (clustered regularly interspaced short palindromic repeats) and CRISPR-associated protein (Cas) have gained extensive applications in bioassays. However, CRISPR-based detection platforms are often hampered by limited analytical sensitivity, while nucleic acid-based amplification strategies are usually indispensable for additional signal enhancement with potential risks of amplification leakages. To address these challenges, an amplification-free CRISPR-based bioassay of aflatoxin B1 (AFB1) was proposed by applying single nanoparticle counting. Single-particle mode inductively coupled plasma mass spectrometry (Sp-ICPMS) has been regarded as a sensitive tool for nanoparticle counting since one nanoparticle can generate considerable signals above backgrounds. With AFB1, activator strands were introduced to initiate the trans-cleavage of CRISPR/Cas12a for cutting the nanoparticles-tagged-magnetic beads, which were transduced to nanoparticle count signals after separation. Finally, a pico-mole level limit-of-detections (LODs) with moderate selectivity was achieved. Certified reference materials (CRMs) analysis and recovery tests were conducted with promising results. To our best knowledge, this is the first report of the single particle counting-based CRISPR/Cas12a biosensing study.

Label-free detection of biomolecules using inductively coupled plasma mass spectrometry (ICP-MS)

Bioassays using inductively coupled plasma mass spectrometry (ICP-MS) have gained increasing attention because of the high sensitivity of ICP-MS and the various strategies of labeling biomolecules with detectable metal tags. The classic strategy to tag the target biomolecules is through direct antibody-antigen interaction and DNA hybridization, and requires the separation of the bound from the unbound tags. Label-free ICP-MS techniques for biomolecular assays do not require direct labeling: they generate detectable metal ions indirectly from specific biomolecular reactions, such as enzymatic cleavage. Here, we highlight the development of three main strategies of label-free ICP-MS assays for biomolecules: (1) enzymatic cleavage of metal-labeled substrates, (2) release of immobilized metal ions from the DNA backbone, and (3) nucleic acid amplification-assisted aggregation and release of metal tags to achieve amplified detection. We briefly describe the fundamental basis of these label-free ICP-MS assays and discuss the benefits and drawbacks of various designs. Future research is needed to reduce non-specific adsorption and minimize background and interference. Analytical innovations are also required to confront challenges faced by in vivo applications.

Data analysis for nanoparticles characterization with single particle inductively coupled plasma mass spectrometry: From microsecond to millisecond dwell times

Single particle-inductively coupled plasma-mass spectrometry (SP-ICP-MS) has become a powerful technique for the characterization of nanoparticles (NPs). However, the accuracy of the characterization of NPs by SP-ICP-MS is greatly affected by the data acquisition rate and the way of data processing. For SP-ICP-MS analysis, ICP-MS instruments typically apply microsecond to millisecond dwell times (10 μs-10 ms). Considering the duration of one nanoparticle event in the detector is 0.4-0.9 ms, NPs will show different data forms when working with microsecond and millisecond dwell times. In this work, the effects of dwell times from microsecond to millisecond (50 μs, 100 μs, 1 ms and 5 ms) on the data forms in SP-ICP-MS analysis are discussed. The data analysis and data processing for different dwell times is discussed in detail, including the measurement of transport efficiency (TE), the distinction of signal and background, the evaluation of diameter limit of detection (LOD_d) and the quantification of mass, size and particle number concentration (PNC) of NPs. This work provides data support for the data processing process and aspects to be considered in the characterization of NPs by SP-ICP-MS, which is expected to provide guidance and reference for researchers in SP-ICP-MS analysis.

Ultrasensitive detection of multiple cancer biomarkers by a triple cascade amplification strategy in combination with single particle inductively coupled plasma mass spectrometry

A versatile triple cascade amplification strategy was developed for ultrasensitive simultaneous detection of multiple cancer biomarkers using single particle inductively coupled plasma mass spectrometry (spICP-MS). The triple cascade amplification strategy consisted of an enhanced RecJ_f exonuclease-assisted target recycling amplification module, a hybridization chain reaction amplification module, and a signal amplification module based on DNA-templated multiple metal nanoclusters. In the enhanced RecJ_f exonuclease-assisted target recycling amplification module, the DNA bases at the 5' ends of aptamers for specific recognition of biomarkers were deliberately replaced by the corresponding RNA bases to enhance amplification efficiency. The signal amplification module based on DNA-templated multiple metal nanoclusters was innovatively used to amplify the signals measured by spICP-MS and at the same time effectively suppress possible background interferences. The proposed spICP-MS platform achieved satisfactory quantitative results for both carcinoembryonic antigen (CEA) and a-fetoprotein (AFP) in human serum samples with accuracy comparable to that of the commercial ELISA kits. Moreover, it has wide dynamic ranges for both CEA (0.01-100 ng/mL) and AFP (0.01-200 ng/mL). The limit of detection for CEA and AFP was 0.6 and 0.5 pg/mL, respectively. Compared with conventional biomarkers detection methods, the proposed spICP-MS platform has the advantages of operational simplicity, ultra-high sensitivity, wide dynamic range, and low background. Therefore, it is reasonable to expect that the proposed spICP-MS platform can be further developed to be a promising alternative tool for biomarker detection in fields of clinical diagnosis and biomedical research.

Single-Nanoparticle Differential Immunoassay for Multiplexed Gastric Cancer Biomarker Monitoring

Precision medicine demands the best application of multiple unambiguous biomarkers to bring uniform decisions in disease prognosis. The remarkable development of heterogeneous immunoassay greatly promotes precision medicine when combined with the biomarker combination strategy. Nevertheless, the cumbersome washing steps in heterogeneous immunoassay have inevitably compromised the accuracy because of the sample losses and nature change of the matrix, challenging the further exploration of a more facile and lower limit-of-detection analysis. The new methodologies with high throughputs and specificity are never out of date to provide simultaneous evaluations and uniform decisions on multiple analytes through a simple process. Herein, we propose a new wash-free immunoassay, named differential assay, for multiplexed biomarker monitoring. The method is based on counting the number difference of unbound nanoparticle tags before and after immunoreactions from a solid support (i.e., magnetic microsphere) by single-particle inductively coupled plasma mass spectrometry (sp-ICP-MS), discarding the tedious washing steps. We primarily explore the proof-of-concept proposal within two types (sandwich and competitive assay), demonstrating the good feasibility for further facile clinical practice. To provide efficient multiplexed evaluations, we synthesized PtNPs with four diameters and screened the most suitable size for efficient differential immunoassay. The wash-free strategy was successfully utilized in simultaneous serological biomarker (CA724, CA199, and CEA) evaluation, with results in good accordance with those measured by the clinical routine method. Potentially, the proposed differential bioassay can be regarded as a more facile and valuable tool in malignancy prognosis and cancer recurrence monitoring.

Facets of ICP-MS and their potential in the medical sciences—Part 2: nanomedicine, immunochemistry, mass cytometry, and bioassays

Inductively coupled–plasma mass spectrometry (ICP-MS) has transformed our knowledge on the role of trace and major elements in biology and has emerged as the most versatile technique in elemental mass spectrometry. The scope of ICP-MS has dramatically changed since its inception, and nowadays, it is a mature platform technology that is compatible with chromatographic and laser ablation (LA) systems. Over the last decades, it kept pace with various technological advances and was inspired by interdisciplinary approaches which endorsed new areas of applications. While the first part of this review was dedicated to fundamentals in ICP-MS, its hyphenated techniques and the application in biomonitoring, isotope ratio analysis, elemental speciation analysis, and elemental bioimaging, this second part will introduce relatively current directions in ICP-MS and their potential to provide novel perspectives in the medical sciences. In this context, current directions for the characterisation of novel nanomaterials which are considered for biomedical applications like drug delivery and imaging platforms will be discussed while considering different facets of ICP-MS including single event analysis and dedicated hyphenated techniques. Subsequently, immunochemistry techniques will be reviewed in their capability to expand the scope of ICP-MS enabling analysis of a large range of biomolecules alongside elements. These methods inspired mass cytometry and imaging mass cytometry and have the potential to transform diagnostics and treatment by offering new paradigms for personalised medicine. Finally, the interlacing of immunochemistry methods, single event analysis, and functional nanomaterials has opened new horizons to design novel bioassays which promise potential as assets for clinical applications and larger screening programs and will be discussed in their capabilities to detect low-level proteins and nucleic acids.

Single Particle Inductively Coupled Plasma Mass Spectrometry-Based Homogeneous Detection of HBV DNA with Rolling Circle Amplification-Induced Gold Nanoparticle Agglomeration

A highly sensitive and simple method based on rolling circle amplification (RCA) and single particle inductively coupled plasma mass spectrometry (spICP-MS) was proposed for the homogeneous detection of hepatitis B virus (HBV) deoxyribonucleic acid (DNA). In the presence of target DNA, long ssDNA possessing a large number of repeating sequence units was generated by RCA. DNA-labeled AuNP probes assembled into long chains based on complementary base pairing, further aggregating into large particles. Small Au NPs hardly produced pulse signals in spICP-MS; obvious pulse signals appeared in spICP-MS after the agglomeration of Au NPs caused by the addition of RCA products and spermidine. On the basis of this, the homogeneous detection of target DNA was realized by spICP-MS with high sensitivity. Under optimal conditions, the proposed method exhibited a good linear relationship between the frequency of the pulse signal of Au in spICP-MS and the concentration of target HBV DNA in the range of 10-2000 fmol L^-1 (R = 0.997), the limit of detection was 5.1 fmol L^-1, and the relative standard deviation was 3.7-6.8%. Recoveries of 94.2-108% were obtained for target DNA in spiked serum samples, demonstrating a good matrix tolerance ability for the method.

Agarose-Droplet-Based Digital LAMP Assay for Counting Virus DNA in Single-Particle ICP-MS

Inductively coupled plasma mass spectrometry (ICP-MS) has emerged as a promising analytical platform for the quantification of biomolecules using elemental tags; however, absolute quantification at extremely low concentrations by ICP-MS without a calibration curve remains challenging. Here, we developed a digital loop-mediated isothermal amplification (LAMP) assay for counting hepatitis B virus (HBV) DNA using single-particle (sp) ICP-MS. The sample and LAMP reagents were mixed and encapsulated in agarose droplets, which were generated by homemade centrifugal droplet generators. The agarose droplets were incubated at 65 °C for amplifying the virus DNA with LAMP primers and then cooled to 4 °C for generating "gel" particles during the temperature-dependent "sol-gel" transition. The LAMP amplicons were intercalated into the agarose particles using polyacrylamide-modified LAMP primers, enabling the labeling of dsDNA with [Ru(bpy)₂dppz]²⁺ and the removal of excess reagents. Only those agarose particles, containing virus DNA, could be labeled with ¹⁰¹Ru and detected in spICP-MS. We also embedded the ¹⁵³Eu-containing polystyrene microspheres into agarose droplets as the internal standard for counting the total number of agarose droplets. The copy number of virus DNA could be counted from the ¹⁰¹Ru/¹⁵³Eu pulse numbers in spICP-MS. We achieved the lowest quantification of 25 copy μL^-1 virus DNA in one analysis without the need for a calibration curve. The developed assay can be easily tuned for counting multiple types of nucleic acid targets and extended for new possibilities of the spICP-MS-based digital assay.

A homogeneous nucleic acid assay for simultaneous detection of SARS-CoV-2 and influenza A (H3N2) by single-particle inductively coupled plasma mass spectrometry

In recent years, single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) has become a powerful tool for biological quantitative analysis. Homogeneous analysis method requires no separation and washing steps, which is suited for the analysis of highly infectious pathogens, so as to reduce the risk of infection during the operation. SARS-CoV-2 spreads all over the world, and its early infection symptoms are similar to influenza, which brings inconvenience to triage. Therefore, developing novel analytical method for simultaneous detection of multiple viral nucleic acids is essential. Taking the advantages of SP-ICP-MS and homogeneous analysis strategy, a SP-ICP-MS homogeneous nucleic acid assay by using gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs) probes was established for simultaneous sensitive analysis of SARS-CoV-2 and influenza A (H3N2). In the present of target SARS-CoV-2 or H3N2 nucleic acids, corresponding Au NPs or Ag NPs probes form larger aggregates, resulting in increased pulse signal intensity and reduced pulse signal frequency of the corresponding NPs in SP-ICP-MS measurement. In this assay, the reaction system of Au NPs and Ag NPs probes does not interfere with each other, and there was no separation and washing procedure, which facilitates operation, saves the analysis time, and improves the analysis efficiency. The linear range of this method is 5-1000 pmol L^-1, with low-level limits of quantification of target nucleic acid. The developed SP-ICP-MS simultaneous homogeneous detection method has a good potential for detecting nucleic acid, protein, cell and other biological samples by changing different modification sequences on the NPs probes.

Mass Spectrometric Biosensing: A Powerful Approach for Multiplexed Analysis of Clinical Biomolecules

Rapid and sensitive detection of clinical biomolecules in a multiplexed fashion is of great importance for accurate diagnosis of diseases. Mass spectrometric (MS) approaches are exceptionally suitable for clinical analysis due to its high throughput, high sensitivity, and reliable qualitative and quantitative capabilities. To break through the bottleneck of MS technique for detecting high-molecular-weight substances with low ionization efficiency, the concept of mass spectrometric biosensing has been put forward by adopting mass spectrometric chips to recognize the targets and mass spectrometry to detect the signals switched by the recognition. In this review, the principle of mass spectrometric sensing, the construction of different mass tags used for biosensing, and the typical combination mode of mass spectrometric imaging (MSI) technique are summarized. Future perspectives including the design of portable matching platforms, exploitation of novel mass tags, development of effective signal amplification strategies, and standardization of MSI methodologies are proposed to promote the advancements and practical applications of mass spectrometric biosensing.

Lanthanide Nanoprobes for the Multiplex Evaluation of Breast Cancer Biomarkers

Metal stable isotope tagging has demonstrated great and unique success in the multiplex and ratiometry-based accurate detection of biomolecules and single cells, while its sensitivity is regarded as an Achilles' heel. Although lanthanide nanoparticles remain the most promising tags for elemental mass spectrometry, there is no report on the lanthanide nanoparticle-based multiplex immunoassay of disease markers in clinical serum samples because of their tough synthesis and bioconjugation and a complex physiological sample matrix. Herein, to fill this gap, multiple lanthanide nanoparticle tags (NaEuF₄, NaTbF₄, and NaHoF₄) were delicately designed and facilely synthesized with a one-pot solvothermal method for the multiplex evaluation of breast cancer biomarkers carcinoembryonic antigen (CEA), CA153, and CA125 in human serum samples. The proposed method exhibited wide linear ranges and low levels of the detection limit for all biomarkers. The test results were consistent with the routine electrochemiluminescence results in clinical serum samples, which proved the possibility of the early prognosis of breast cancer as well as improving the surgical outcome prediction.

Quantification of Tumor Protein Biomarkers from Lung Patient Serum Using Nanoimpact Electrochemistry

Protein quantification with high throughput and high sensitivity is essential in the early diagnosis and elucidation of molecular mechanisms for many diseases. Conventional approaches for protein assay often suffer from high costs, long analysis time, and insufficient sensitivity. The recently emerged nanoimpact electrochemistry (NIE), as a contrast, allows in situ detection of analytes one at a time with simplicity, fast response, high throughput, and the potential of reducing the detection limits down to the single entity level. Herein, we propose a NIE-enabled electrochemical immunoassay using silver nanoparticles (AgNPs) as labels for the detection of CYFRA21-1, a typical protein marker for lung carcinoma. This strategy is based on the measurement of the impact frequency and the charge intensity of the electrochemical oxidation of individual AgNPs before and after they are modified with anti-CYFRA21-1 and in turn immunocomplexed with CYFRA21-1. Both the frequency and intensity modes of single-nanoparticle electrochemistry correlate well with each other, resulting in a self-validated immunoassay that provides linear ranges of two orders of magnitude and a limit of detection of 0.1 ng/mL for CYFRA21-1 analysis. The proposed immunoassay also exhibits excellent specificity when challenged with other possible interfering proteins. In addition, the CYFRA21-1 content is validated by a conventional, well-known enzyme-linked immunosorbent assay and successfully quantified in a diluted healthy serum with a satisfactory recovery. Moreover, CYFRA21-1 detection in serum samples of lung cancer patients is successfully demonstrated, suggesting the feasibility of the NIE-based immunoassay in clinically relevant diagnosis. To the best of our knowledge, this is the first report to construct NIE-based electrochemical immunoassays for the specific detection of tumor protein biomarkers.

Single-Nanoparticle ICP-MS for Sensitive Detection of Uracil-DNA Glycosylase Activity

Single-nanoparticle inductively coupled plasma mass spectrometry (SP-ICP-MS) has demonstrated unique advantages for the detection of biological samples. However, methods for enzyme activity detection based on SP-ICP-MS technology have been rarely explored. Here we report the development of a novel SP-ICP-MS assay for uracil-DNA glycosylase (UDG) activity detection based on its ability to specifically recognize and remove uracil to induce the cleavage of the DNA probe. Our design allows the generation of single gold nanoparticles correlated to the specific enzymatic reaction for a highly sensitive SP-ICP-MS measurement. The developed assay enables sensitive UDG activity detection with a detection limit of 0.0003 U/mL. The cell lysate analysis by the developed assay reveals its applicability for the detection of UDG activity in real samples. It is envisioned that our design may provide a new paradigm for developing the SP-ICP-MS assay for enzyme activity detection in biological samples.

A Homogeneous Multicomponent Nucleic Acid Enzyme Assay for Universal Nucleic Acid Detection by Single-Particle Inductively Coupled Plasma Mass Spectrometry

Single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS) has great potential for sensitive analysis of nucleic acids; however, it usually requires separation of target-induced nanoparticle reporters, and the sequence of probes on nanoparticle reporters has to be tuned for each target accordingly. Here, we developed a homogeneous multicomponent nucleic acid enzyme (MNAzyme) assay for universal nucleic acid detection. The two components of MNAzyme contain target recognition sites, substrate binding sites, and a catalytic core. Only in the presence of a specific nucleic acid target, the MNAzyme will assemble to trigger its nucleic acid enzyme activity and cleave its substrate (Linker DNA). The Linker DNA could link gold nanoparticle (AuNP) probes to form a larger assembled particle, while the cleavage of Linker DNA will disturb the linkage between probes, inducing a smaller assembled particle. The assembled particles with different sizes could be differentiated and sensitively detected in SP-ICP-MS, which also enables the tolerance of a complex matrix. By simply altering the sequences of the target recognition sites in MNAzyme, we applied the assay for two types of nucleic acids (long strand DNA and short strand RNA), malaria DNA and miRNA-10b. With increasing the target concentration, the signal intensity of each assembled particle decreases, but the frequency of assembled particle pulse increases. Both targets could be quantitatively detected from 0.1 to 25 pmol L^-1 with high specificity in serum samples. The developed MNAzyme-SP-ICP-MS assay possesses simple operation in a homogeneous reaction, easy tunability for multiple types of nucleic acid targets, and good compatibility with clinic samples.

Noble-metal nanoparticle labelling multiplex miRNAs by ICP-MS readout with internal standard isotopes of 115In and 209Bi

Inductively coupled plasma-mass spectrometry (ICP-MS) is one of the most powerful techniques for multiplex nucleotide assay owing to the virtue of the high resolution of multiple-elements' mass to charge ratio, in a mass spectrum. Here, a small sized (less than 20 nm) noble-metal nanoparticle labelled ICP-MS (NP-ICP-MS) is proposed for high-throughput microRNA (miRNA) determination. Three miRNA targets - miR-486-5p, miR-221, and miR-21 - in serum, were distinguished by single-stranded DNA (ssDNA) probes labelled with a small sized noble-metal nanoparticle - silver nanoparticles (AgNPs), platinum nanoparticles (PtNPs), and gold nanoparticles (AuNPs). The counting isotopes ion intensity per second (CPS) of the noble-metal label versus internal standard isotope intensity of ¹¹⁵In and ²⁰⁹Bi, exhibited good linearity in the range 0.25 pM to 100 pM with correlation coefficients (R²) of 0.9680, 0.9305, and 0.9418. The specific sandwich-type miRNA assay using the sensitive NP-ICP-MS readout pushed the detection limits down to 0.18 pM for miR-221, 0.23 pM for miR-486-5p, and 0.22 pM for miR-21. And the relative standard deviations (RSDs) for 10 pM target miRNA were less than 3.7%. This work promises a potential ultrasensitive ICP-MS bioassay of multiplex miRNA biomarkers for clinical serum diagnosis.

A novel miniaturized homogeneous label-free electrochemical biosensing platform combining integrated microelectrode and functional nucleic acids

A miniaturized platform combining integrated microelectrode (IME) and functional nucleic acids was developed for homogeneous label-free electrochemical biosensing. IME was constructed with a carbon fiber microelectrode and a platinum wire in a θ type glass tube as a two-electrode system for electrochemical monitoring at microliter level. A newly reported G-triplex/methylene blue (G3/MB) complex was used as the signal generator in the homogeneous label-free electrochemical biosensor. G3 has strong affinity with MB and it can cause significant decrease of the diffusion current of MB after binding. Melamine was chosen as the model target. Since melamine can interact with nucleobase thymine (T) to form T-melamine-T structure through complementary hydrogen bonds, a single-strand functional DNA hairpin structure with poly T and G3 elaborately blocked via base pairing was designed. The presence of melamine can trigger the conformation switching of the DNA hairpin to release the G3. The released G3 combined with MB could therefore change the diffusion current, leading to a simple and rapid detection of melamine. The combination of functional DNA hairpin as target recognition element, G3/MB as signal generator, and IME as transducer provided a "Mix and Measure" miniaturized platform for the construction of homogeneous label-free electrochemical biosensors.

Signal amplification strategies for clinical biomarker quantification using elemental mass spectrometry

The current trends in modern medicine towards early diagnosis, or even prognosis, of different diseases have brought about the need for the corresponding biomarker detection at ever lower levels in really complex matrices. To do so, it is necessary to use proper extremely sensitive detection techniques such as elemental mass spectrometry. However, target labelling with metals for subsequent sensitive ICP-MS detection falls short nowadays even if resorting to inorganic nanoparticles containing a high number of detectable elements. Thus, new amplification strategies are being proposed to face this analytical challenge that will be critically discussed in this paper. Fundamentals of different novel strategies developed to achieve signal amplification and sensitive elemental mass spectrometry detection are here discussed. Some representative examples of relevant clinical applications are highlighted, along with future prospects and challenges.

Graphene/gold nanoparticle composites for ultrasensitive and versatile biomarker assay using single-particle inductively-coupled plasma/mass spectrometry

An ultrasensitive and versatile assay for biomarkers has been developed using graphene/gold nanoparticles (AuNPs) composites and single-particle inductively-coupled plasma/mass spectrometry (spICP-MS). Thrombin was chosen as a model biomarker for this study. AuNPs modified with thrombin aptamers were first non-selectively adsorbed onto the surface of graphene oxide (GO) to form GO/AuNPs composites. In the presence of thrombin, the AuNPs desorbed from the GO/AuNPs composites due to a conformation change of the thrombin aptamer after binding with thrombin. The desorbed AuNPs were proportional to the concentration of thrombin and could be quantified by spICP-MS. By counting the individual AuNPs in the spICP-MS measurement, the concentration of thrombin could be determined. This assay achieved an ultralow detection limit of 4.5 fM with a broad linear range from 10 fM to 100 pM. The method also showed excellent selectivity and reproducibility when a complex protein matrix was evaluated. Furthermore, the diversity and ready availability of ssDNA ligands make this method a versatile new technique for ultrasensitive detection of a wide variety of biomarkers in clinical diagnostics.

Homogeneous assay based on the pre-reduction and selective cation exchange for detection of multiple targets by atomic spectrometry

In view of the high sensitivity and good selectivity, chemical vapor generation atomic spectrometry (CVG-AS) and inductively coupled plasma mass spectrometer (ICP-MS), especially low-cost atomic fluorescence spectrometry (AFS) have been widely used in bioassay. However, the existing AS method is mostly based on heterogeneous strategies, and can't detect multiple targets in one system. In this study, we present the discovery and mechanism study of a phenomenon of Hg²⁺ pre-reduction that the concentration of Hg²⁺ decreased when it was mixed with the reductants (ascorbic acid (AA), SnCl₂, or NaBH₄/KBH₄) over long-time reaction (hours) by CVG-AFS and ICP-MS. A homogeneous Cu²⁺ assay method was developed based on the competition reaction of Cu²⁺ and Hg²⁺ for consuming AA, and its application in the detection of pyrophosphate (PPi) and alkaline phosphatase (ALP) was investigated based on the PPi complexation with Cu²⁺, and ALP hydrolyzation of PPi using CVG-AFS as a representative detector. Subsequently, in order to further verify the applicability of the system, cation exchange reaction (CER) was utilized here based on the selectively recognize Ag⁺ and C-Ag⁺-C by CuS nanoparticles (NPs). As the exchanged Cu²⁺ from CuS NPs can be sensitively and selectively detected via above-mentioned Cu²⁺ assay method, this strategy can be extended for the Ag⁺, DNA and prostate specific antigen (PSA) detection based on base complementary pairing and the specific recognition of aptamer. Under the optimal experimental conditions, the system showed high sensitivity for the detection of Cu²⁺, PPi, ALP, Ag⁺, DNA, and PSA, with limit of detections (LODs) of 0.12 nmol L^-1, 25 μmol L^-1, 0.025 U/L, 0.2 nmol L^-1, 0.05 nmol L^-1, and 0.03 ng/mL, respectively. The method was successfully used to determination Cu²⁺, ALP, and PSA in human serums, showing similar results with those of ICP-MS and kits methods.

Thiol inhibition of Hg cold vapor generation in SnCl2/NaBH4 system: A homogeneous bioassay for H2O2/glucose and butyrylcholinesterase/pesticide sensing by atomic spectrometry

Recently, the use of atomic spectrometry (AS) for biochemical analysis has attracted considerable attention due to its high sensitivity, selectivity and anti-interference ability. In this work, we conducted a detailed study on a phenomenon of thiol inhibition of mercury (Hg²⁺) cold vapor generation (CVG) and found _L-cysteine (_L-Cys), glutathione (GSH), dithiothreitol, N-Acetyl-L-cysteine, 3-mercaptopropionic acid, β-mercaptoethanol, and NaI can inhibit the CVG of Hg²⁺, while EDTA has no inhibitory effect. Furthermore, changing the content of -SH can effectively adjust the CVG atomic fluorescence spectrometer (CVG-AFS) signal of Hg²⁺. As as a consequence, an AS-based homogeneous bioassay was constructed by adjusting the oxidation ratio and production quantity of -SH in the system. The quantitative analysis of the system was demonstrated by using AFS as a representative detector. Hydrogen peroxide (H₂O₂) and glucose were used as representative analytes for the validation of Hg²⁺ atomic fluorescence signal turn-off strategy, and butyrylcholinesterase (BChE) as well as parathion (organophosphorus pesticides, OPs) as utilized as representative targets for the signal turn-on strategy. Under optimal experimental conditions, the homogeneous CVG-AFS sensor can be successfully used to detect 3 μM H₂O₂, 30 μM glucose, 0.25 U/L BChE, and 0.4 μg/mL parathion. In addition, the detection results of glucose and BChE in human serum samples agreed well with those obtained by using glucometer and kit, showing the promising potential of this method for practical applications. Therefore, this work provides a perspective for the construction of AS-based homogeneous bioassays and shows great potential for the detection of biomarkers.

Self-Validated Homogeneous Immunoassay by Single Nanoparticle in-Depth Scrutinization

The most convenient method for the clinical routine analysis of disease biomarkers is homogeneous immunoassay, which minimizes the requirements for automation and time-/lab-consumption. Despite great success, because sample constituents are not removed by a separation or washing step, a major challenge in conducting homogeneous immunoassays for the practical application is the matrix effect-related inaccuracy. Herein, to guarantee an accurate quantification, a self-validated homogeneous immunoassay was proposed, by simultaneously scrutinizing both frequency and intensity of single gold nanoparticles. The two analytical modes of single particle inductively coupled plasma mass spectrometry (ICPMS) correlated well with each other, resulting in a self-validation mechanism for the accurate immunoassay. Both two modes of the proposed method provided linear ranges of 2 orders of magnitude and LODs of pM level. Thanks to the self-validated strategy and the high tolerance of the matrix effect of ICPMS, the proposed homogeneous immunoassay was successfully demonstrated in a series of human serum samples, with results in good accordance with clinical routine methods.

Aggregation-based determination of mercury(II) using DNA-modified single gold nanoparticle, T-Hg(II)-T interaction, and single-particle ICP-MS

An ultrasensitive assay is described for the detection and determination of Hg²⁺(aq) in water samples based on single-particle inductively-coupled plasma/mass spectrometry (spICP-MS). In the presence of Hg²⁺(aq), AuNPs modified with a segment of single-stranded DNA aggregate due to the formation of the well-known thymine (T)-Hg²⁺-T complex. Single particle (sp) ICP-MS is used quantify the degree of aggregation by the overall decrease in number of detected AuNPs or NP aggregates. Compared with most other Hg²⁺ assays that use the same principle of aggregation-dispersion with DNA modified AuNPs, this method has a much lower detection limit of (0.031 ng L^-1, 155 fM) and a wider (10,000-fold) linear range (up to 1 μg L^-1). The method also showed good practical potential because of its minimal interference from the water sample matrix. Graphical abstractSchematic representation of Hg²⁺ determination by using modified AuNP probes measured by spICP-MS. AuNPs pulses detected in ICP-MS is relative to the aggregation status of AuNPs based on thymine-Hg²⁺-thymine interaction.

Absolute quantification of particle number concentration using a digital single particle counting system

The accurate determination of the molar concentration or the number concentration of particles in a defined volume is important but challenging. Since particle diversity and heterogeneity cannot be ignored in particle quantification, single particle counting has become quite important. However, most methods require standard samples (calibrators) which are usually difficult to obtain. The authors describe a method for single particle counting that is based on the combination of digital counting and formation of microdroplets in a microchip. By compartmentalizing particles into picoliter droplets, positive droplets encapsulating particles were counted and particle concentrations were calculated by Poisson statistics. The concentration of particles over a wide range (from 5.0 × 10³ to 1.8 × 10⁷ particles per mL) were accurately determined without the need for using a calibrator. A microdroplet chip including a T-junction channel achieved a 9-fold increase of signal-to-background ratio compared to the traditional flow-focusing chip. This makes the digital counting system a widely applicable tool for quantification of fluorescent particles. Various particles including differently sized fluorescent microspheres and bacteria with large heterogeneity in shape such as Escherichia coli DH5α-pDsRed were accurately quantified by this method. Graphical abstract Schematic representation of the digital single particle counting system for absolute quantification of particles. Particles compartmentalized in picoliter droplets were counted and the number concentration of particles was determined using digital analysis.

Single particle ICP-MS-based absolute and relative quantification of E. coli O157 16S rRNA using sandwich hybridization capture

Herein, a novel 16S rRNA detection platform was achieved by combining a sandwich hybridization reaction, a single-molecule magnetic capture, and single particle-inductively coupled plasma mass spectrometry amplification. The assay was developed for the direct detection of RNA from dangerous human pathogens and enabled absolute and high-precision quantification of a target with a detection limit of 10 fM.

Simultaneous determination of two phosphorylated p53 proteins in SCC-7 cells by an ICP-MS immunoassay using apoferritin-templated europium(III) and lutetium(III) phosphate nanoparticles as labels

Phosphorylated p53 proteins are biomarkers with clinical utility for early diagnosis of cancer, but difficult to quantify. An inductively coupled plasma mass spectrometry (ICP-MS) based immunoassay is described here that uses uniform lanthanide nanoparticles (NPs) as elemental tags for the simultaneous determination of two phosphorylated p53 proteins. Apoferritin templated europium (Eu) phosphate (AFEP) NPs and apoferritin templated lutetium (Lu) phosphate (AFLP) NPs with 8 nm in diameter were used to label two phosphorylated p53 proteins at serine 15 and serine 392 sites (p-p53¹⁵ and p-p53³⁹²), respectively. The assay has a sandwich format, and p-p53¹⁵ and p-p53³⁹² were first captured and then recognized by AFEP and AFLP NPs labelled antibodies, respectively. The Eu and Lu were then released from the immune complexes under acidic condition for ICP-MS measurement. The limits of detection for p-p53¹⁵ and p-p53³⁹² are 200 and 20 pg·mL^-1, with linear ranges of 0.5-20 and 0.05-20 ng·mL^-1, respectively. The method was further applied to study the response of p-p53¹⁵ and p-p53³⁹² in SCC-7 cells exposed to the natural carcinogen arsenite. A significant up-regulation of p-p53¹⁵ and p-p53³⁹² can be observed when cells were exposed to arsenite at 5 μmol·L^-1 level for 24 h. Graphical abstract Schematic presentation of the ICP-MS immunoassay using apoferritin templated europium (III) and lutetium (III) phosphate nanoparticles as labels for the simultaneous determination of two phosphorylated p53 proteins. Europium (Eu) phosphate nanoparticles (blue) and lutetium (Lu) phosphate nanoparticles (pink) were synthesized in the size-restricted cavity of apoferritin. They were further coupled with antibodies to prepare Eu and Lu labelled probes for p-p53¹⁵ (blue) and p-p53³⁹² (pink), respectively. After formation of a a sandwich, the labelled Eu and Lu were dissociated in acid and then introduced to ICP-MS for the simultaneous determination of two phosphorylated p53 proteins p-p53¹⁵ (blue) and p-p53³⁹² (pink).

Visual/CVG-AFS/ICP-MS multi-mode and label-free detection of target nucleic acids based on a selective cation exchange reaction and enzyme-free strand displacement amplification

Visual/CVG-AFS/ICP-MS three-mode detection of DNA based on the selective cation exchange reaction and enzyme-free strand displacement amplification.

Applications of Gold Nanoparticles in Non-Optical Biosensors

Due to their unique properties, such as good biocompatibility, excellent conductivity, effective catalysis, high density, and high surface-to-volume ratio, gold nanoparticles (AuNPs) are widely used in the field of bioassay. Mainly, AuNPs used in optical biosensors have been described in some reviews. In this review, we highlight recent advances in AuNP-based non-optical bioassays, including piezoelectric biosensor, electrochemical biosensor, and inductively coupled plasma mass spectrometry (ICP-MS) bio-detection. Some representative examples are presented to illustrate the effect of AuNPs in non-optical bioassay and the mechanisms of AuNPs in improving detection performances are described. Finally, the review summarizes the future prospects of AuNPs in non-optical biosensors.

Metal-isotope-tagged monoclonal antibodies for high-dimensional mass cytometry

Advances in single-cell mass cytometry have increasingly improved highly multidimensional characterization of immune cell heterogeneity. The immunoassay multiplexing capacity relies on monoclonal antibodies labeled with stable heavy-metal isotopes. To date, a variety of rare-earth elements and noble and post-transition metal isotopes have been used in mass cytometry; nevertheless, the methods used for antibody conjugation differ because of the individual metal coordination chemistries and distinct stabilities of various metal cations. Herein, we provide three optimized protocols for conjugating monoclonal IgG antibodies with 48 high-purity heavy-metal isotopes: (i) 38 isotopes of lanthanides, 2 isotopes of indium, and 1 isotope of yttrium; (ii) 6 isotopes of palladium; and (iii) 1 isotope of bismuth. Bifunctional chelating agents containing coordinative ligands of monomeric DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) or polymeric pentetic acid (DTPA) were used to stably sequester isotopic cations in aqueous solutions and were subsequently coupled to IgG antibodies using site-specific biorthogonal reactions. Furthermore, quantification methods based on antibody inherent absorption at 280 nm and on extrinsic absorption at 562 nm after staining with bicinchoninic acid (BCA) are reported to determine metal-isotope-tagged antibodies. In addition, a freeze-drying procedure to prepare palladium isotopic mass tags is described. To demonstrate the utility, experiments using six palladium-tagged CD45 antibodies for barcoding assays of live immune cells in cytometry by time-of-flight (CyTOF) are described. Conjugation of pure isotopes of lanthanides, indium, or yttrium takes ~3.5 h. Conjugation of bismuth takes ~4 h. Preparation of palladium mass tags takes ~8 h. Conjugation of pure isotopes of palladium takes ~2.5 h. Antibody titration takes ~4 h.

Atomic mass tag of bismuth-209 for increasing the immunoassay multiplexing capacity of mass cytometry

Mass cytometry (or CyTOF) is an atomic mass spectrometry-based single-cell immunoassay technology, which has provided an increasingly systematic and sophisticated view in basic biological and clinical studies. Using elemental reporters composed of stable heavy metal isotopes, more than 50 cellular parameters are measured simultaneously. However, this current multiplexing does not meet the theoretical capability of CyTOF instrumentation with 135 detectable channels, primarily due to the limitation of available chemistries for conjugating elemental mass tags to affinity reagents. To address this issue, we develop herein additional metallic mass tag based on bismuth-209 (²⁰⁹ Bi) for efficient conjugation to monoclonal antibody. This enables the use of an addtional channel m/z = 209 of CyTOF for single-cell immunoassays. Bismuth has nearly the same charge-to-radius ratio as lanthanide elements; thus, bismuth(III) cations (²⁰⁹ Bi³⁺ ) could coordinate with DTPA chelators in the same geometry of O- and N-donor groups as that of lanthanide. In this report, the coordination chemistry of ²⁰⁹ Bi³⁺ with DTPA chelators and Maxpar® X8 polymers were investigated in details. Accordingly, the protocols of conjugating antibody with bismuth mass tag were provided. A method based on UV-Vis absorbance at 280 nm of ²⁰⁹ Bi³⁺ -labeling DTPA complexes was developed to evaluate the stoichiometric ratio of ²⁰⁹ Bi³⁺ cations to the conjugated antibody. Side-by-side single-cell analysis experiments with bismuth- and lanthanide-tagged antibodies were carried out to compare the analytical sensitivities. The measurement accuracy of bismuth-tagged antibody was validated within in vitro assay using primary human natural killer cells. Furthermore, bismuth-tagged antibodies were successfully employed in cell cycle measurements and high-dimensional phenotyping immunoassays. © 2017 International Society for Advancement of Cytometry.

Nanotechnology-Enhanced No-Wash Biosensors for in Vitro Diagnostics of Cancer

In vitro biosensors have been an integral component for early diagnosis of cancer in the clinic. Among them, no-wash biosensors, which only depend on the simple mixing of the signal generating probes and the sample solution without additional washing and separation steps, have been found to be particularly attractive. The outstanding advantages of facile, convenient, and rapid response of no-wash biosensors are especially suitable for point-of-care testing (POCT). One fast-growing field of no-wash biosensor design involves the usage of nanomaterials as signal amplification carriers or direct signal generating elements. The analytical capacity of no-wash biosensors with respect to sensitivity or limit of detection, specificity, stability, and multiplexing detection capacity is largely improved because of their large surface area, excellent optical, electrical, catalytic, and magnetic properties. This review provides a comprehensive overview of various nanomaterial-enhanced no-wash biosensing technologies and focuses on the analysis of the underlying mechanism of these technologies applied for the early detection of cancer biomarkers ranging from small molecules to proteins, and even whole cancerous cells. Representative examples are selected to demonstrate the proof-of-concept with promising applications for in vitro diagnostics of cancer. Finally, a brief discussion of common unresolved issues and a perspective outlook on the field are provided.

Single-Step DNA Detection Assay Monitoring Dual-Color Light Scattering from Individual Metal Nanoparticle Aggregates

Efficiently detecting DNA sequences within a limited time is vital for disease screening and public health monitoring. This calls for a new method that combines high sensitivity, fast read-out time, and easy manipulation of the sample, avoiding the extensive steps of DNA amplification, purification, or grafting to a surface. Here, we introduce photon cross-correlation spectroscopy as a new method for specific DNA sensing with high sensitivity in a single-step homogeneous solution phase. Our approach is based on confocal dual-color illumination and detection of the scattering intensities from individual silver nanoparticles and gold nanorods. In the absence of the target DNA, the nanoparticles move independently and their respective scattering signals are uncorrelated. In the presence of the target DNA, the probe-functionalized gold and silver nanoparticles assemble via DNA hybridization with the target, giving rise to temporal coincidence between the signals scattered by each nanoparticle. The degree of coincidence accurately quantifies the amount of target DNA. To demonstrate the efficiency of our technique, we detect a specific DNA sequence of sesame, an allergenic food ingredient, for a range of concentration from 5 pM to 1.5 nM with a limit of detection of 1 pM. Our method is sensitive and specific enough to detect single nucleotide deletion and mismatch. With the dual-color scattering signals being much brighter than fluorescence-based analogs, the analysis is fast, quantitative, and simple to operate, making it valuable for biosensing applications.

A combinatorial immunoassay for multiple biomarkersviaa stable isotope tagging strategy

A combinatorial immunoassay method for biomarker detection based on a stable isotope tagging strategy was proposed.

Metal Stable Isotope Tagging: Renaissance of Radioimmunoassay for Multiplex and Absolute Quantification of Biomolecules

The unambiguous quantification of biomolecules is of great significance in fundamental biological research as well as practical clinical diagnosis. Due to the lack of a detectable moiety, the direct and highly sensitive quantification of biomolecules is often a "mission impossible". Consequently, tagging strategies to introduce detectable moieties for labeling target biomolecules were invented, which had a long and significant impact on studies of biomolecules in the past decades. For instance, immunoassays have been developed with radioisotope tagging by Yalow and Berson in the late 1950s. The later languishment of this technology can be almost exclusively ascribed to the use of radioactive isotopes, which led to the development of nonradioactive tagging strategy-based assays such as enzyme-linked immunosorbent assay, fluorescent immunoassay, and chemiluminescent and electrochemiluminescent immunoassay. Despite great success, these strategies suffered from drawbacks such as limited spectral window capacity for multiplex detection and inability to provide absolute quantification of biomolecules. After recalling the sequences of tagging strategies, an apparent question is why not use stable isotopes from the start? A reasonable explanation is the lack of reliable means for accurate and precise quantification of stable isotopes at that time. The situation has changed greatly at present, since several atomic mass spectrometric measures for metal stable isotopes have been developed. Among the newly developed techniques, inductively coupled plasma mass spectrometry is an ideal technique to determine metal stable isotope-tagged biomolecules, for its high sensitivity, wide dynamic linear range, and more importantly multiplex and absolute quantification ability. Since the first published report by our group, metal stable isotope tagging has become a revolutionary technique and gained great success in biomolecule quantification. An exciting research highlight in this area is the development and application of the mass cytometer, which fully exploited the multiplexing potential of metal stable isotope tagging. It realized the simultaneous detection of dozens of parameters in single cells, accurate immunophenotyping in cell populations, through modeling of intracellular signaling network and undoubted discrimination of function and connection of cell subsets. Metal stable isotope tagging has great potential applications in hematopoiesis, immunology, stem cells, cancer, and drug screening related research and opened a post-fluorescence era of cytometry. Herein, we review the development of biomolecule quantification using metal stable isotope tagging. Particularly, the power of multiplex and absolute quantification is demonstrated. We address the advantages, applicable situations, and limitations of metal stable isotope tagging strategies and propose suggestions for future developments. The transfer of enzymatic or fluorescent tagging to metal stable isotope tagging may occur in many aspects of biological and clinical practices in the near future, just as the revolution from radioactive isotope tagging to fluorescent tagging happened in the past.