Nanomaterial based advancement in the inorganic pyrophosphate detection methods in the last decade: A review

Ratiometric fluorescent sensing of pyrophosphate with sp³-functionalized single-walled carbon nanotubes

Inorganic pyrophosphate is a key molecule in many biological processes from DNA synthesis to cell metabolism. Here we introduce sp3-functionalized (6,5) single-walled carbon nanotubes (SWNTs) with red-shifted defect emission as near-infrared luminescent probes for the optical detection and quantification of inorganic pyrophosphate. The sensing scheme is based on the immobilization of Cu2+ ions on the SWNT surface promoted by coordination to covalently attached aryl alkyne groups and a triazole complex. The presence of Cu2+ ions on the SWNT surface causes fluorescence quenching via photoinduced electron transfer, which is reversed by copper-complexing analytes such as pyrophosphate. The differences in the fluorescence response of sp3-defect to pristine nanotube emission enables reproducible ratiometric measurements in a wide concentration window. Biocompatible, phospholipid-polyethylene glycol-coated SWNTs with such sp3 defects are employed for the detection of pyrophosphate in cell lysate and for monitoring the progress of DNA synthesis in a polymerase chain reaction. This robust ratiometric and near-infrared luminescent probe for pyrophosphate may serve as a starting point for the rational design of nanotube-based biosensors.

Selective pyrophosphate detection via metal complexes

Pyrophosphate (PPi) anions are crucial in numerous biological and ecological processes involved in energy conversion, enzymatic reactions, and metabolic regulation along with adenosine. They are also significant biological markers for various processes related to diseases. Fluorescent PPi sensors would enable visual and/or biological detection in convenient settings. However, the current availability of commercial sensors has been limited to costly enzymes that are not compatible for imaging. Sensor development has also encountered challenges such as poor selectivity and stability, and limited practical applications. In this review, we analyze the situation of PPi sensing via commercial kits and focus on sensors that use metal complexes. We address their designs, sensing mechanisms, selectivities and detection limits. Finally, we discuss limitations and perspectives for PPi detection and imaging.

Double protein directed synthesis of chemically etched sulfur doped quantum dots for signal "on-off-on" sensing of glutathione mediated by copper ions

In this study, a novel "on-off-on" fluorescent probe was suggested for sensitive and selective assay of glutathione (GSH). The as-fabricated nanoswitch employs a Cu2+-sulfur quantum dot system (SQ-dots/Cu2+). The surface reactivity and water solubility of SQ-dots were improved through capping with egg white and bovine serum albumin proteins. The surface functional groups on the surface of double protein-protected SQ-dots enhanced the interaction with Cu2+ ions, resulting in the aggregation induced quenching of SQ-dots. Addition of GSH, a strong Cu2+ ion chelator, disassembles the large aggregates into relatively smaller ones, restoring the fluorescence emission of SQ-dots. Under optimized conditions, the fluorescence intensity was increased by increasing GSH amounts within the range of 0.13-550 μM with a detection limit (S/N = 3) of 0.04 μM. The SQ-dots/Cu2+ system was successfully applied for the detection of GSH in different matrices such as dietary supplements, human serum, and vegetable extract samples. The as-fabricated probe holds great potential for the synthesis of other functionalized SQ-dots for (bio) sensing.

Protein-directed synthesis of fluorescent sulfur quantum dots for highly robust detection of pyrophosphate

Inorganic pyrophosphate anions (PPi) play a key role in various biological processes and act as an essential indicator for physiological function evaluation and disease diagnosis. However, there is still a lack of available approaches for straightforward, robust, and convenient PPi detection. Herein, we design an on-off-on fluorescent switching nanoprobe employing Fe³⁺-mediated fluorescent sulfur quantum dots (SQDs) for highly robust detection of PPi. The bovine serum protein (BSA)-capped SQDs with fine water dispersibility and good optical stability are synthesized by an H₂O₂-assisted chemical etching reaction. Specifically, Fe³⁺ can strongly induce the aggregation of the SQDs into relatively larger sizes, resulting in aggregation-induced fluorescence quenching behavior. PPi can selectively bind with Fe³⁺ via emulative coordination and in preventing the aggregation of SQDs this is accompanied by recovery of fluorescence. The physicochemical properties of aggregated and disaggregated SQDs have been systematically investigated. Aggregation and disaggregation of the SQDs and the corresponding quenching and recovery of fluorescence occurs and guarantees the high-contrast sensing performance of the SQD system in complex and challenging aquatic environments. Our designed on-off-on nanoswitch holds great potential for the design of elemental quantum dot-based biosensors for the highly robust detection of analytes in the near future.

Triazol-Methanaminium-Pillar[5]arene-Functionalized Single Nanochannel for Quantitative Analysis of Pyrophosphate in Water

Inorganic pyrophosphate (PPi) is an important biological functional anion and plays crucial roles in life science, environmental science, medicine, and chemical process. Quantification of PPi in water has far-reaching significance for life exploration, disease diagnosis, and water pollution control. The label-free quantitative detection of PPi anions with a nanofluidic sensing device based on a conical single nanochannel is demonstrated. The channel surface is functionalized with a synthetic PPi receptor, triazol-methanaminium-functionalized pillar[5]arene (TAMAP5), using carbodiimide coupling chemistry. Due to the specific binding between TAMAP5 and PPi, the functionalized nanochannel can discriminate PPi from other inorganic anions with high selectivity through ionic current recording, even in the presence of various interfering anions. The current response exhibits a linear correlation with PPi concentration in the range from 1 × 10^-7 to 1 × 10^-4 M with a limit of detection of 6.8 × 10^-7 M. A spike-and-recovery analysis of PPi in East Lake water samples indicates that the proposed nanofluidic sensor has the ability to quantitate micromolar concentrations of PPi in environmental water samples.