Enzyme-free amplified DNA assay: five orders of linearity provided by metal stable isotope detection

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.

Highly Sensitive and Specific Mass Spectrometric Platform for miRNA Detection Based on the Multiple-Metal-Nanoparticle Tagging Strategy

The multiple-metal-nanoparticle tagging strategy has generally been applied to the multiplexed detection of multiple analytes of interest such as microRNAs (miRNAs). Herein, it was used for the first time to improve both the specificity and sensitivity of a novel mass spectroscopic platform for miRNA detection. The mass spectroscopic platform was developed through the integration of the ligation reaction, hybridization chain reaction amplification, multiple-metal-nanoparticle tagging, and inductively coupled plasma mass spectrometry. The high specificity resulted from the adoption of the ligation reaction is further enhanced by the multiple-metal-nanoparticle tagging strategy. The combination of hybridization chain reaction amplification and metal nanoparticle tagging endows the proposed platform with the feature of high sensitivity. The proposed mass spectrometric platform achieved quite satisfactory quantitative results for Let-7a in real-world cell line samples with accuracy comparable to that of the real-time quantitative reverse-transcriptase polymerase chain reaction method. Its limit of detection and limit of quantification for Let-7a were experimentally determined to be about 0.5 and 10 fM, respectively. Furthermore, due to the unique way of utilizing the multiple-metal-nanoparticle tagging strategy, the proposed platform can unambiguously discriminate between the target miRNA and nontarget ones with single-nucleotide polymorphisms based on their response patterns defined by the relative mass spectral intensities among the multiple tagged metal elements and can also provide location information of the mismatched bases. Its unique advantages over conventional miRNA detection methods make the proposed platform a promising and alternative tool in the fields of clinical diagnosis and biomedical research.

A nanoprobe based on molybdenum disulfide nanosheets and silver nanoclusters for imaging and quantification of intracellular adenosine triphosphate

Adenosine triphosphate (ATP), as a high-energy phosphate compound that stores and releases energy in living cells, has an irreplaceable role in many physiological processes and maintenance of biological functions, and can be used as an indicator of many diseases. In this work, a composite nanoprobe, silver nanocluster (AgNC) @ molybdenum disulfide (MoS₂), was designed to achieve in situ fluorescence imaging and quantitative analysis of intracellular ATP in HeLa cells by fluorescence spectrometry and inductively coupled plasma mass spectrometry (ICP-MS). The probe was constructed based on the adsorption of DNA-AgNCs by MoS₂ nanosheets, and the DNA-AgNCs were prepared with the ATP aptamer as a template, whose fluorescence was initially quenched by MoS₂. When the probe was incubated into the cells, intracellular ATP recognized the aptamer sequence and caused the DNA-AgNCs to fall off the MoS₂ nanosheets, resulting in fluorescence recovery. Here, AgNCs not only acted as a fluorescence label for imaging, but also as an element tag for quantitative analysis of intracellular ATP with the detection of ¹⁰⁷Ag by ICP-MS. The ATP in HeLa cells detected by this method was 24.6 ± 1.7 nmol L^-1, which was in good agreement with the test result of the ATP test kit (20.4 ± 0.8 nmol L^-1). The proposed method has potential application in medical clinical diagnosis and evaluation of the body's metabolic level via fluorescence imaging and ICP-MS detection of intracellular ATP.

Dually enhanced homogenous synthesis of molybdophosphate by hybridization chain reaction and enzyme nanotags for the electrochemical bioassay of carcinoembryonic antigen

A magnetic bead (MB)-based sandwich biorecognition reactions is combined with a gold nanoprobe-induced homogenous synthesis of molybdophosphate to develop a novel bioassay method for the electrochemical detection of the tumor biomarker of carcinoembryonic antigen (CEA). The nanoprobe is prepared through the specific loading of numerous alkaline phosphatase (ALP)-functionalized gold nanoparticles (Au NPs) on a double-stranded DNA (dsDNA) produced by the CEA aptamer-triggered hybridization chain reaction (HCR). Both the large amounts of PO₄^3- produced by the ALP catalytic hydrolysis of pyrophosphate and the phosphate backbones of dsDNA can react with the added MoO₄^2- to generate electroactive molybdophosphates. So, the gold nanoprobe was used for signal tracing of the sandwich bioassay of CEA at a constructed antibody-functionalized MB platform. The sensitive electrochemical measurement of molybdophosphate produced from the quantitatively captured nanoprobes at a carbon nanotube-modified electrode (measured at about 0.12 V vs. Ag/AgCl, 3 M KCl) enabled the convenient signal transduction of the method. Due to the dually enhanced synthesis of molybdophosphate by the HCR and multi-enzyme Au NP nanotags, this method shows a wide linear range from 0.05 pg mL^-1 to 10 ng mL^-1 along with a low detection limit of 0.027 pg mL^-1. In addition, the MB-based biorecognition reaction and the homogeneous synthesis of molybdophosphate are much convenient in manipulations. These excellent performances decide the extensive application potentials of the method. Graphical abstract A magnetic bead-based bioassay method was simply developed for the electrochemical detection of carcinoembryonic antigen. The dually enhanced homogenous synthesis of molybdophosphate by hybridization chain reaction (HCR) and enzyme nanotags and the sensitive electrochemical measurement of molybdophosphate at a carbon nanotube (CNT)-electrode enable ultrasensitive signal transduction of the method.