Lanthanide-Coordinated Semiconducting Polymer Dots Used for Flow Cytometry and Mass Cytometry

Metal-detection based techniques and their applications in metallobiology

Metals are essential for human health and play a crucial role in numerous biological processes and pathways. Gaining a deeper insight into these biological events will facilitate novel strategies for disease prevention, early detection, and personalized treatment. In recent years, there has been significant progress in the development of metal-detection based techniques from single cell metallome and proteome profiling to multiplex imaging, which greatly enhance our comprehension of the intricate roles played by metals in complex biological systems. This perspective summarizes the recent progress in advanced metal-detection based techniques and highlights successful applications in elucidating the roles of metals in biology and medicine. Technologies including machine learning that couple with single-cell analysis such as mass cytometry and their application in metallobiology, cancer biology and immunology are also emphasized. Finally, we provide insights into future prospects and challenges involved in metal-detection based techniques, with the aim of inspiring further methodological advancements and applications that are accessible to chemists, biologists, and clinicians.

Steric Hindrance-Mediated Enzymatic Reaction Enable Homogeneous Dual Fluorescence Indicators Aptasensing of Hepatocellular Carcinoma CTCs

Circulating tumor cells (CTCs) serve as important biomarkers in the liquid biopsy of hepatocellular carcinoma (HCC). Herein, a homogeneous dual fluorescence indicators aptasensing strategy is described for CTCs in HCC, with the core assistance of a steric hindrance-mediated enzymatic reaction. CTCs in the sample could specifically bind to a 5'-biotin-modified glypican-3 (GPC3) aptamer and remove the steric hindrance formed by the biotin-streptavidin system. This influences the efficiency of the terminal deoxynucleotidyl transferase enzymatic reaction. Then, methylene blue (MB) was introduced to react with the main product poly cytosine (polyC) chain, and trivalent cerium ion (Ce3+) was added to react with the byproduct pyrophosphate to form fluorescent pyrophosphate cerium coordination polymeric nanoparticles. Finally, the CTCs were quantified by dual fluorescence indicators analysis. Under optimized conditions, the linear range was 5 to 104 cells/mL, and the limits of detection reached 2 cells/mL. Then, 40 clinical samples (15 healthy and 25 HCC patients) were analyzed. The receiver operating characteristic curve analysis revealed an area under the curve of 0.96, a sensitivity of 92%, and a specificity of 100%. Therefore, this study established a sensitive and accurate CTCs sensing system for clinical HCC patients, promoting early tumor diagnosis.

Platinum-Chimeric Carrier Cells for Ultratrace Cell Analysis in Mass Cytometry

Mass cytometry by time-of-flight (CyTOF), a high-dimensional single-cell analysis platform, detects up to 50 biomarkers at single-cell resolution. However, CyTOF analysis of biological samples with a minimal number of available cells or rare cell subsets remains a major technical challenge due to the extensive loss of cells during cell recovery, staining, and acquisition. Here, we introduce a platinum-chimeric carrier cell strategy for mass cytometry profiling of ultratrace cell samples. Cisplatin can rapidly enter broken plasma membranes of dead cells and form a chimeric interaction with cellular proteins, peptides, and amino acids. Thus, 198Pt-cisplatin is adopted to tag carrier cells in the pretreatment stage. We investigated 8 cell lines that are commonly accessible in laboratories for their potential as carrier cells to preserve rare target cells for CyTOF analysis. We designed a panel of 35 protein biomarkers to evaluate the comprehensive single-cell subtype classification capability with or without the carrier cell strategy. We further demonstrated the detection and analysis of as few as 1 × 104 immune cells using our method. The proposed method thus allows CyTOF analysis on precious clinical samples with less abundant cells.

Iron-Rich Semiconducting Polymer Dots for the Combination of Ferroptosis-Starvation and Phototherapeutic Cancer Therapy

Chemodynamic therapy (CDT) has emerged as an outstanding antitumor therapeutic method due to its selectivity and utilization of tumor microenvironment. However, there are still unmet requirements to achieve a high antitumor efficiency, including the tumor accumulation of catalyst and enrichment of reactants of Fenton reaction. Here, an iron-loaded semiconducting polymer dot modified with glucose oxidase (Pdot@Fe@GOx) is reported to deliver iron ions into tumor tissues and in situ generation of hydrogen peroxide in tumors. On one hand, Pdot@Fe@GOx converts glucose to gluconic acid and hydrogen peroxide (H2 O2 ) in tumor, which not only consumes glucose of tumor cells, but also provides the H2 O2 for the following Fenton reaction. On the other hand, the Pdot@Fe@GOx delivers active iron ions in tumor to perform CDT with the combination of the generated H2 O2 . In addition, the Pdot@Fe@GOx has both photothermal and photodynamic effects under the irradiation of near-infrared laser, which can improve and compensate the CDT effect to kill cancer cells. This Pdot@Fe@GOx-based multiple-mode therapeutic strategy has successfully achieved a synergistic anticancer effect with minimal side effects and has the potential to be translated into preclinical setting for tumor therapy.

A universal mass tag based on polystyrene nanoparticles for single-cell multiplexing with mass cytometry

Mass cytometry (MC) is an emerging bioanalytical technique for high-dimensional biomarkers interrogation simultaneously on individual cells. However, the sensitivity and multiplexed analysis ability of MC was highly restricted by the current metal chelating polymer (MCP) mass tags. Herein, a new design strategy for MC mass tags by using a commercial available and low cost classical material, polystyrene nanoparticle (PS-NP) to carry metals was reported. Unlike inorganic materials, sub-micron-grade metal-loaded polystyrene can be easily detected by MC, thus it is not essential to pursue extremely small particle size in this mass tag design strategy. An altered cell staining buffer can significantly lower the nonspecific binding (NSB) of non-functionalized PS-NPs, revealing another method to lower NSB beside surface modification. The metal doped PS-NP_Abs mass tags showed high compatibility with MCP mass tags and 5-fold higher sensitivity. By using Hf doped PS-NP_Abs as mass tags, four new MC detection channels (¹⁷⁷Hf, ¹⁷⁸Hf, ¹⁷⁹Hf and ¹⁸⁰Hf) were developed. In general, this work provides a new strategy in designing MC mass tags and lowering NSB, opening up possibility of introducing more potential MC mass tag candidates.

Mass tag-encoded nanointerfaces for multiplexed mass spectrometric analysis and imaging of biomolecules

Revealing multiple biomolecules in the physiopathological environment simultaneously is crucial in biological and biomedical research. Mass spectrometry (MS) features unique technical advantages in multiplexed and label-free analyses. However, owing to comparably low abundance and poor ionization efficiency of target biomolecules, direct MS profiling of these biological species in vitro or in situ remains a challenge. An emerging route to solve this issue is to devise mass tag (MT)-encoded nanointerfaces which specifically convert the abundance or activity of biomolecules into amplified ion signals of mass tags, offering an ideal strategy for synchronous MS assaying and mapping of multiple targets in biofluids, cells and tissues. This review provides a thorough and organized overview of recent advances in MT-encoded nanointerfaces elaborately tailored for several practical applications in multiplexed MS bioanalysis and biomedical research. First, we start with elucidation of the structural characteristics and working principle of MT-encoded nanointerfaces in specific labeling and sensing of multiple biological targets. In addition, we further discuss the application scenarios of MT-encoded nanointerfaces particularly in multiplexed biomarker assays, cell analysis, and tissue imaging. Finally, the current challenges are pointed out and future prospects of these nanointerfaces in MS analysis are forecast.

Reagents for Mass Cytometry

Mass cytometry (cytometry by time-of-flight detection [CyTOF]) is a bioanalytical technique that enables the identification and quantification of diverse features of cellular systems with single-cell resolution. In suspension mass cytometry, cells are stained with stable heavy-atom isotope-tagged reagents, and then the cells are nebulized into an inductively coupled plasma time-of-flight mass spectrometry (ICP-TOF-MS) instrument. In imaging mass cytometry, a pulsed laser is used to ablate ca. 1 μm² spots of a tissue section. The plume is then transferred to the CyTOF, generating an image of biomarker expression. Similar measurements are possible with multiplexed ion bean imaging (MIBI). The unit mass resolution of the ICP-TOF-MS detector allows for multiparametric analysis of (in principle) up to 130 different parameters. Currently available reagents, however, allow simultaneous measurement of up to 50 biomarkers. As new reagents are developed, the scope of information that can be obtained by mass cytometry continues to increase, particularly due to the development of new small molecule reagents which enable monitoring of active biochemistry at the cellular level. This review summarizes the history and current state of mass cytometry reagent development and elaborates on areas where there is a need for new reagents. Additionally, this review provides guidelines on how new reagents should be tested and how the data should be presented to make them most meaningful to the mass cytometry user community.

Hybrid Fluorescent Mass-Tag Nanotrackers as Universal Reagents for Long-Term Live-Cell Barcoding

Barcoding and pooling cells for processing as a composite sample are critical to minimize technical variability in multiplex technologies. Fluorescent cell barcoding has been established as a standard method for multiplexing in flow cytometry analysis. In parallel, mass-tag barcoding is routinely used to label cells for mass cytometry. Barcode reagents currently used label intracellular proteins in fixed and permeabilized cells and, therefore, are not suitable for studies with live cells in long-term culture prior to analysis. In this study, we report the development of fluorescent palladium-based hybrid-tag nanotrackers to barcode live cells for flow and mass cytometry dual-modal readout. We describe the preparation, physicochemical characterization, efficiency of cell internalization, and durability of these nanotrackers in live cells cultured over time. In addition, we demonstrate their compatibility with standardized cytometry reagents and protocols. Finally, we validated these nanotrackers for drug response assays during a long-term coculture experiment with two barcoded cell lines. This method represents a new and widely applicable advance for fluorescent and mass-tag barcoding that is independent of protein expression levels and can be used to label cells before long-term drug studies.

Highly sensitive and multiplexed mass spectrometric immunoassay techniques and clinical applications

Immunoassay is one of the most important clinical techniques for disease/pathological diagnosis. Mass spectrometry (MS) has been a popular and powerful readout technique for immunoassays, generating the mass spectrometric immunoassays (MSIAs) with unbeatable channels for multiplexed detection. The sensitivity of MSIAs has been greatly improved with the development of mass labels from element labels to small-molecular labels. MSIAs are also expended from the representative element MS-based methods to the laser-based organic MS and latest ambient MS, improving in both technology and methodology. Various MSIAs present high potential for clinical applications, including the biomarker screening, the immunohistochemistry, and the advanced single-cell analysis. Here, we give an overall review of the development of MSIAs in recent years, highlighting the latest improvement of mass labels and MS techniques for clinical immunoassays.

Dynamic metabolic change of cancer cells induced by natural killer cells at single-cell level studied by label-free mass cytometry

Natural killer cells (NK cells) are important immune cells which have attracted increasing attention in cancer immunotherapy. Due to the heterogeneity of cells, individual cancer cells show different resistance to NK cytotoxicity, which has been revealed by flow cytometry. Here we used label-free mass cytometry (CyESI-MS) as a new tool to analyze the metabolites in Human Hepatocellular Carcinoma (HepG2) cells at the single-cell level after the interaction with different numbers of NK92 MI cells. A large amount of chemical information from individual HepG2 cells was obtained showing the process of cell apoptosis induced by NK cells. Nineteen metabolites which consecutively change during cell apoptosis were revealed by calculating their average relative intensity. Four metabolic pathways were impacted during cell apoptosis which hit 4 metabolites including glutathione (GSH), creatine, glutamic acid and taurine. We found that the HepG2 cells could be divided into two phenotypes after co-culturing with NK cells according to the bimodal distribution of concentration of these 4 metabolites. The correlation between metabolites and different apoptotic pathways in the early apoptosis cell group was established by the 4 metabolites at the single-cell level. This is a new idea of using single-cell specific metabolites to reveal the metabolic heterogeneity in cell apoptosis which would be a powerful means for evaluating the cytotoxicity of NK cells.

Label-free mass cytometry is utilized to study the dynamic metabolic change during apoptosis in HepG2 cells induced by NK92 MI cells at the single-cell level. The metabolic heterogeneity of individual HepG2 cells during apoptosis was revealed.

New Structure Mass Tag based on Zr-NMOF for Multiparameter and Sensitive Single-Cell Interrogating in Mass Cytometry

Mass cytometry, also called cytometry by time-of-flight (CyTOF), is an emerging powerful proteomic analysis technique that utilizes metal chelated polymer (MCP) as mass tags for interrogating high-dimensional biomarkers simultaneously on millions of individual cells. However, under the typical polymer-based mass tag system, the sensitivity and multiplexing detection ability has been highly restricted. Herein, a new structure mass tag based on a nanometal organic framework (NMOF) is reported for multiparameter and sensitive single-cell biomarker interrogating in CyTOF. A uniform-sized Zr-NMOF (33 nm) carrying 105 metal ions is synthesized under modulator/reaction time coregulation, which is monodispersed and colloidally stable in water for over one-year storage. On functionalization with an antibody, the Zr mass tag exhibits specific molecular recognition properties and minimal cross-reaction toward nontargeted cells. In addition, the Zr-mass tag is compatible with MCP mass tags in a multiparameter assay for mouse spleen cells staining, which exploits four additional channels, m/z = 90, 91, 92, 94, for single-cell immunoassays in CyTOF. Compared to the MCP mass tag, the Zr-mass tag provides an additional fivefold signal amplification. This work provides the fundamental technical capability for exploiting NMOF-based mass tags for CyTOF application, which opens up possibility of high-dimensional single-cell immune profiling, low abundant antigen detection, and development of new barcoding systems.

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Multiplexed live-cell profiling with Raman probes

Single-cell multiparameter measurement has been increasingly recognized as a key technology toward systematic understandings of complex molecular and cellular functions in biological systems. Despite extensive efforts in analytical techniques, it is still generally challenging for existing methods to decipher a large number of phenotypes in a single living cell. Herein we devise a multiplexed Raman probe panel with sharp and mutually resolvable Raman peaks to simultaneously quantify cell surface proteins, endocytosis activities, and metabolic dynamics of an individual live cell. When coupling it to whole-cell spontaneous Raman micro-spectroscopy, we demonstrate the utility of this technique in 14-plexed live-cell profiling and phenotyping under various drug perturbations. In particular, single-cell multiparameter measurement enables powerful clustering, correlation, and network analysis with biological insights. This profiling platform is compatible with live-cell cytometry, of low instrument complexity and capable of highly multiplexed measurement in a robust and straightforward manner, thereby contributing a valuable tool for both basic single-cell biology and translation applications such as high-content cell sorting and drug discovery.

Measuring Cellular Uptake of Polymer Dots for Quantitative Imaging and Photodynamic Therapy

There is a great deal of interest in the development of nanoparticles for biomedicine. The question of how many nanoparticles are taken up by cells is important for biomedical applications. Here, we describe a fluorescence method for the quantitative measurement of the cellular uptake of polymer dots (Pdots) and a further estimation of intracellular Pdots photosensitizer for fluorescence imaging and photodynamic therapy. The approach relies on the high brightness, excellent stability, minimal aggregation quenching, and metalloporphyrin doping properties of the Pdots. We correlated the single-cell fluorescence brightness obtained from fluorescence spectrometry, confocal microscopy, and flow cytometry with the number of endocytosed Pdots, which was validated by inductively coupled plasma mass spectrometry. Our results indicated that, on average, ∼1.3 million Pdots were taken up by single cells that were incubated for 4 h with arginine 8-Pdots (40 μg/mL, ∼20 nm diameter). The absolute number of endocytosed Pdots of individual cells could be estimated from confocal microscopy by comparing the single-cell brightness with the average intensity. Furthermore, we investigated the cell viability as a result of an intracellular Pdots photosensitizer, from which the half maximal inhibitory concentration was determined to be ∼7.2 × 10⁵ Pdots per cell under the light dose of 60 J/cm². This study provides an effective method for quantifying endocytosed Pdots, which can be extended to investigate the cellular uptake of various conjugated polymer carriers in biomedicine.

Mass Cytometry Tags: Where Chemistry Meets Single-Cell Analysis

Mass cytometry is a highly multiparametric proteomic technology that allows the measurement and quantification of nearly 50 markers with single-cell resolution. Mass cytometry reagents are probes tagged with metal isotopes of defined mass and act as reporters. Metals are detected using inductively coupled plasma time-of-flight mass spectrometry (ICP-TOF-MS). Many different types of mass-tag reagents have been developed to afford myriad applications. We have classified these compounds into polymer-based mass-tag reagents, nonpolymer-based mass-tag reagents, and inorganic nanoparticles. Metal-chelating polymers (MCPs) are widely used to profile and quantify cellular biomarkers; however, both the range of metals that can be detected and the metal signals have to be improved. Several strategies such as the inclusion of chelating agents or highly branched polymers may overcome these issues. Biocompatible materials such as polystyrene and inorganic nanoparticles are also of profound interest in mass cytometry. While polystyrene allows the inclusion of a wide variety of metals, the high metal content of inorganic nanoparticles offers an excellent opportunity to increase the signal from the metals to detect low-abundance biomarkers. Nonpolymer-based mass-tag reagents offer multiple applications: cell detection, cell cycle property determination, biomarker detection, and mass-tag cellular barcoding (MCB). Recent developments have been achieved in live cell barcoding by targeting proteins (CD45, b2m, and CD298), by using small and nonpolar probes or by ratiometric barcoding. From this perspective, the principal applications, strengths, and shortcomings of mass-tag reagents are highlighted, summarized, and discussed, with special emphasis on mass-tag reagents for MCB. Thereafter, the future perspectives of mass-tag reagents are discussed considering the current state-of-the-art technologies.

Engineering fluorescent semiconducting polymer nanoparticles for biological applications and beyond

We summarize the recent advances in engineering approaches to obtain functionalized semiconducting polymer nanoparticles (SPNs) for biological applications. The challenges and outlook of fabricating functionalized SPNs are also provided.

Visual and dual-fluorescence homogeneous sensor for the detection of pyrophosphatase in clinical hyperthyroidism samples based on selective recognition of CdTe QDs and coordination polymerization of Ce3+

A visual / dual fluorescent strategy based on selective recognition of QDs and coordination polymerization of Ce³⁺ was developed for pyrophosphatase detection.

Quantification of cyclic DNA polymerization with lanthanide coordination nanomaterials for liquid biopsy

Quantification of circulating tumor DNA (ctDNA) is of great importance in liquid biopsy but difficult due to its low amount in bodily fluids. To meet this high demand, a novel method for ctDNA detection is established by quantifying cyclic DNA polymerization using lanthanide coordination polymers (Ln-CPs). Relying on the coordination between the pyrophosphate ion (PPi) and trivalent cerium ion (Ce3+), organic ligand-free PPi-Ce coordination polymer networks (PPi-Ce CPNs) with enhanced fluorescence are prepared for the first time. By surveying the optical properties of PPi-Ce CPNs, it is found that PPi regulates electric-dipole transition of Ce3+ to the lowest excited state, thus facilitating the emission of fluorescence. Therefore, fluorescence enhancement of PPi-Ce CPNs originates from the ligand field effect rather than the normal antenna effect. Moreover, a new strategy to quantify DNA polymerization is developed based on PPi-Ce CPNs. By introducing multifold cyclic DNA polymerization, a small amount of ctDNA triggers the exponential generation of PPi to form plenty of PPi-Ce CPNs. Accordingly, a biosensor is constructed for sensitive ctDNA detection by measuring the intense fluorescence of PPi-Ce CPNs. The biosensor is capable of sensing ctDNA at the sub-femtomolar level, which is far better than the analytical performances of commercial dyes. Besides, the analytical method is able to detect single nucleotide polymorphism and determine ctDNA in real samples. Considering that DNA polymerization is widely used in bio-recognition, bio-assembly and biomineralization, the work provides a versatile quantitative strategy of making relevant processes precise and controllable.

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.

Dual resonance energy transfer in triple-component polymer dots to enhance electrochemiluminescence for highly sensitive bioanalysis

Polymer dots (Pdots) have become a type of attractive illuminant for electrochemiluminescence (ECL). However, the low ECL efficiency severely limits their practicability. Here, we design a dual intramolecular resonance energy transfer (RET) mechanism with newly synthesized triple-component Pdots to achieve great ECL enhancement. This mechanism efficiently shortens the path of energy transmission, thus greatly promoting the ECL amplification by 380 and 31 times compared to systems with no and single RET, and results in a relative ECL efficiency of 23.1% (vs. 1 mM Ru(bpy)32+). Using metal-organic frameworks to carry the triple-component Pdots, a highly luminescent probe is proposed. By integrating the probe with target-mediated enzymatic circulation amplification and DNA arrays, a highly sensitive ECL imaging method is designed for simultaneous visual analysis of two kinds of proteins, mucin 1 and human epidermal growth factor receptor 2, on living cells, which exhibited linear ranges of 1 pg mL-1 to 5 ng mL-1 and 5 pg mL-1 to 10 ng mL-1 with limits of detection of 1 pg mL-1 and 5 pg mL-1, respectively. The proposed strategy showed promising application in bioanalysis.

A BODIPY-Based Donor/Donor-Acceptor System: Towards Highly Efficient Long-Wavelength-Excitable Near-IR Polymer Dots with Narrow and Strong Absorption Features

Bright long-wavelength-excitable semiconducting polymer dots (LWE-Pdots) are highly desirable for in vivo imaging and multiplexed in vitro bioassays. LWE-Pdots have been obtained by incorporating a near-infrared (NIR) emitter into the backbone of a polymer host to develop a binary donor-acceptor (D-A) system. However, they usually suffer from severe concentration quenching and a trade-off between fluorescence quantum yield (Φf ) and absorption cross-section (σ). Herein, we describe a ternary component (D1 /D2 -A) strategy to achieve ultrabright, green laser-excitable Pdots with narrow-band NIR emission by introducing a BODIPY-based assistant polymer donor as D1 . The D1 /D2 -A Pdots possess improved Φf and σ compared to corresponding binary D2 -A Pdots. Their Φf is as high as 40.2 %, one of the most efficient NIR Pdots reported. The D1 /D2 -A Pdots show ultrahigh single-particle brightness, 83-fold brighter than Qdot 705 when excited by a 532 nm laser. When injected into mice, higher contrast in vivo tumor imaging was achieved using the ternary Pdots versus the binary D-A Pdots.

Chemical-Modified Nucleotide-Based Elemental Tags for High-Sensitive Immunoassay

Multiplex biomolecular analysis with inductively coupled plasma mass spectrometry (ICP-MS) becomes increasingly important in clinical diagnosis and single cell analysis. However, the sensitivity of ICP-MS-based immunoassay is only comparable or lower than those of fluorescence methods at the present stage. Therefore, designing elemental tags with a large number of metal atoms is necessary to achieve high-sensitive detection. In this work, we proposed a new strategy to build up elemental tag loading with hundreds of rare earth ions by coupling alkyne-DNA chains with rare earth element (REE)-DOTA complexes a click chemistry reaction. There are about 2 orders of magnitude improvement in sensitivity compared with single metal-ion tags. DNA chains with multialkynyl groups were facilely prepared by PCR synthesis. Moreover, the DNA-based elemental tags own excellent water-solubility and biocompatibility. The tags would be potentially applied to mass cytometry and clinical diagnosis.

Lanthanide nanoparticles for high sensitivity multiparameter single cell analysis

Mass cytometry (MC) is a high throughput multiparameter analytical technique for determining biomarker expression in cells. In MC, antibodies (Abs) are tagged with heavy metal isotopes via conjugation to metal chelating polymers (MCPs). To improve the sensitivity of MC towards low abundance biomarkers, we are developing nanoparticle (NP)-based reagents as mass tags for Abs. We examine the use of silica-coated NaHoF4 NPs (d ∼ 12 nm) decorated with PEG5k conjugated to thiol-modified primary or secondary Abs for MC assays. We compare the sensitivity of NP-Ab conjugates to MCP-Ab conjugates towards seven biomarkers with varying expression levels across six cell lines. We also perform a multi-parameter assay using a cocktail of both NP- and MCP-based reagents to detect seven cellular markers in peripheral blood mononuclear cells (PBMCs). In the case of highly abundant markers, signal enhancements from NP-Ab conjugates offer minimal advantages over MCP-Ab conjugates, which already give strong signals. In the case of biomarkers with lower abundance, the level of signal enhancements depended on the nature of the biomarker being detected, or on the type of detection method used. When comparing the indirect detection of CD14 on THP-1 cells using NPs or MCPs conjugated to secondary Abs, the NP reagents offered little signal enhancements compared to the MCP reagents. However, in the case of direct CD14 detection on THP-1 or U937 cells using NPs or MCPs conjugated to primary Abs, a 30- or 450-fold signal enhancement was seen from the NP-based reagent. In the experiments where both NP-Ab and MCP-Ab conjugates were used together to stain PBMCs, we found that the presence of the NP-Ab conjugates did not affect the function of MCP-Ab conjugates, and the NP-Ab conjugates showed minimal non-specific interaction with cells without the target biomarker (CD14). Furthermore, these NP-Ab conjugates could be used to identify rare CD14+ monocytes from the PBMC mixture with a 20-fold signal increase when compared to the use of only MCP-Ab conjugates. Collectively, the strong signal amplification obtained from NP reagents demonstrate the potential of these reagents to be used in conjunction with MCP-reagents to detect rare cellular markers or cell types that may otherwise be overlooked when using MCP-reagents alone.

Identification of Human Antigen-Specific CD4+ T-Cells with Peptide-MHC Multimer Technologies

The development of peptide-MHC class II multimers has provided a novel approach in studying antigen-specific CD4⁺ T-cells and extended the knowledge of these T cells in various disease settings, including infectious diseases, autoimmune diseases, cancer, and allergies. This chapter discusses the various applications of the peptide-MHC class II multimer technologies, specifically their uses in the evaluation of antigen-specific CD4⁺ T-cells ex vivo, and their uses in epitope identification.

Modular synthesis and modification of novel bifunctional dendrons

The modular synthesis of two generations of highly branched bifunctional dendrons is reported. The first generation dendron–antibody conjugate is shown to selectively detect CD4⁺ T cells in the PBMC culture.

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.

Ratiometric Barcoding for Mass Cytometry

Barcoding is of importance for high-throughput cellular and molecular analysis. A ratiometric barcoding strategy using lanthanide-coordinated polymer dots (Ln-Pdots) was developed for mass cytometric analysis. By using 3 metal isotopes and 4 ratio intensity levels, 16 barcodes were generated to code, and later decode, cell samples in mass cytometry. The ratiometric Ln-Pdot barcodes not only provided high-mass-signal intensities but also eliminated the bias caused by different concentrations of the labeling reagents/barcodes and run-to-run differences in cell labeling efficiency. The ability to distinguish clearly the 16 sets of labeled MCF-7 cells with mass cytometry demonstrated the excellent resolving power of the ratiometric Ln-Pdot barcodes. Furthermore, the results from barcoding PBMC samples via CD45-specific cellular targeting indicated that the ratiometric Ln-Pdot barcodes could facilitate mass cytometry in high-throughput and multiplexed analysis, especially with precious human samples.

Clickable and High-Sensitivity Metal-Containing Tags for Mass Cytometry

Mass cytometry is a highly multiplexed single-cell analysis platform that uses metal-tagged reagents to identify multiple cellular biomarkers. The current metal-tagged reagent preparation employs thiol-maleimide chemistry to covalently couple maleimide-functionalized metal-chelating polymers (MCPs) with antibodies (Abs), a process that requires partial reduction of the Ab to form reactive thiol groups. However, some classes of Abs (for example, IgM) as well as biomolecules lacking cysteine residues have been challenging to label using this method. This inherent limitation led us to develop a new conjugation strategy for labeling a wide range of biomolecules and affinity reagents. In this report, we present a metal tagging approach using a new class of azide- or transcyclooctene-terminated MCPs with copper(I)-free strain-promoted alkyne-azide cycloaddition or tetrazine-alkene click chemistry reactions, in which biomolecules with -NH₂ functional groups are selectively activated with a dibenzocyclooctyne or tetrazine moiety, respectively. This approach enabled us to generate highly sensitive and specific metal-tagged IgGs, IgMs, small peptides, and lectins for applications in immunophenotyping and glycobiology. We also created dual-tagged reagents for simultaneous detection of markers by immunofluorescence, mass cytometry, and imaging mass cytometry using a two-step conjugation process. The Helios mass cytometer was used to test the functionality of reagents on suspension human leukemia cell lines and primary cells. The dual-tagged Abs, metal-tagged lectins, and phalloidin staining reagent were used to visualize target proteins and glycans on adherent cell lines and frozen/FFPE tissue sections using the Hyperion Imaging System. In some instances, reagents produced by click conjugation showed superior sensitivity and specificity compared to those of reagents produced by thiol-maleimide chemistry. In general, the click chemistry-based conjugation with new MCPs could be instrumental in developing a wide range of highly sensitive metal-containing reagents for proteomics and glycomics applications.