An overview on the development of different optical sensing platforms for adenosine triphosphate (ATP) recognition

Adenosine triphosphate (ATP), one of the biological anions, plays a crucial role in several biological processes including energy transduction, cellular respiration, enzyme catalysis and signaling. ATP is a bioactive phosphate molecule, recognized as an important extracellular signaling agent. Apart from serving as a universal energy currency for various cellular events, ATP is also considered a factor responsible for numerous physiological activities. It regulates cellular metabolism by breaking phosphoanhydride bonds. Several diseases have been reported widely based on the levels and behavior of ATP. The variation of ATP concentration usually causes a foreseeable impact on mitochondrial physiological function. Mitochondrial dysfunction is responsible for the occurrence of many severe diseases such as angiocardiopathy, malignant tumors and Parkinson's disease. Therefore, there is high demand for developing a sensitive, fast-responsive, nontoxic and versatile detection platform for the detection of ATP. To this end, considerable efforts have been employed by several research groups throughout the world to develop specific and sensitive detection platforms to recognize ATP. Although a repertoire of optical chemosensors (both colorimetric and fluorescent) for ATP has been developed, many of them are not arrayed appropriately. Therefore, in this present review, we focused on the design and sensing strategy of some chemosensors including metal-free, metal-based, sequential sensors, aptamer-based sensors, nanoparticle-based sensors etc. for ATP recognition via diverse binding mechanisms.

Cu2+-mediated turn-on fluorescence biosensor based on DNA-templated silver nanoclusters for label-free and sensitive detection of adenosine triphosphate

A Cu²⁺-mediated turn-on fluorescence biosensor based on the DNA-templated green-emitting silver nanoclusters (DNA@g-AgNCs) was developed for label-free and sensitive detection of adenosine 5'-triphosphate (ATP). Cu²⁺ was able to quench the bright green fluorescence of DNA@g-AgNCs because of the coordination and photoinduced electron transfer between DNA@g-AgNCs and Cu²⁺. Therefore, a unique and effective fluorescence biosensor can be constructed with the formation of DNA@g-AgNCs/Cu²⁺/ATP ternary-competition system. With the introduction of ATP, the DNA@g-AgNCs/Cu²⁺ fluorescence sensing system will be disrupted and the fluorescence of DNA@g-AgNCs was recovered due to higher affinity of ATP towards Cu²⁺. On the basis of this feature, the DNA@g-AgNCs/Cu²⁺ fluorescence sensing system demonstrated quantitative determination of ATP in the range 0.05 - 3 μM and a detection limit of 16 nM. Moreover, the fluorescence sensing system was successfully applied to the quantitative determination of ATP in human urine and serum samples with recoveries ranging from 98.6 to 106.5%, showing great promise to provide  a label-free, cost-efficient, and rapid platform for ATP-related clinical disease diagnosis.

Detection of Pyrophosphate and Alkaline Phosphatase Activity Based on PolyT Single Stranded DNA - Copper Nanoclusters

The determination of pyrophosphate and alkaline phosphatase activity plays a significant role in medical diagnosis. In this work, a label-free "ON-OFF-ON" fluorescence strategy is developed for the analysis of pyrophosphate and alkaline phosphatase activity. Using PolyT single strand DNA as templates to synthesize fluorescent copper nanoparticles, the coordination effect of pyrophosphoric acid on Cu²⁺ inhibited the generation of fluorescence. Afterwards, the addition of alkaline phosphatase into hydrolyze pyrophosphoric acid resulted in the release of Cu^2+, whereby the fluorescence intensity could be recovered. Thereupon enhanced-sensitivity for alkaline phosphatase was obtained (0.1 mU/L), much better than previously reported methods. Meanwhile, it could be performed directly in homogeneous solution, which was very close to the actual activity level of alkaline phosphatase under physiological conditions. Likewise, satisfactory results were also obtained in specificity assessment, which demonstrated its potential application in clinical diagnosis. Notably, a new, sensitive, low-cost, short-time, and high-sensitivity platform for alkaline phosphatase detection was constructed, and the design of biosensor using DNA-templated Copper nanoclusters (CuNCs) was instructed in this study.

A carbon dot doped lanthanide coordination polymer nanocomposite as the ratiometric fluorescent probe for the sensitive detection of alkaline phosphatase activity

The development of sensitive methods for alkaline phosphatase (ALP) activity analysis is an important analytical topic. Based on the stimulus-responsive lanthanide coordination polymer, a simple ratiometric fluorescence sensing strategy was proposed to detect ALP activity. A carbon dot (CD) doped fluorescent supramolecular lanthanide coordination polymer (CDs@Tb-GMP) was prepared with Tb³⁺ and the ligand guanine single nucleotide (GMP). To construct a ratiometric fluorescence biosensor, the fluorescence of Tb-GMP was used as a response signal, and the fluorescence of CDs was used as a reference signal due to its good stability. When excited at 290 nm, the polymer network Tb-GMP emits characteristic fluorescence at 545 nm, while the CDs encapsulated in the polymer network emit fluorescence at 370 nm. After adding ALP to the system, the substrate GMP can be hydrolyzed by ALP, resulting in the destruction of the polymer network. Accordingly, the fluorescence of Tb-GMP significantly decreased, while the fluorescence of CDs slightly increased due to their release from the polymer network. By comparing the relationship between the fluorescence intensity ratio of the two signals and the concentration of ALP, sensitive detection of ALP could be achieved with the linear range from 0.5 to 80 U L^-1 and a detection limit of 0.13 U L^-1. Furthermore, the proposed ratiometric sensing system was applied to the detection of ALP in human serum samples with desirable results, indicating potential application in clinical diagnosis.

Long Wavelength TCF-Based Fluorescent Probe for the Detection of Alkaline Phosphatase in Live Cells

A long wavelength TCF-based fluorescent probe (TCF-ALP) was developed for the detection of alkaline phosphatase (ALP). ALP-mediated hydrolysis of the phosphate group of TCF-ALP resulted in a significant fluorescence "turn on" (58-fold), which was accompanied by a colorimetric response from yellow to purple. TCF-ALP was cell-permeable, which allowed it to be used to image ALP in HeLa cells. Upon addition of bone morphogenic protein 2, TCF-ALP proved capable of imaging endogenously stimulated ALP in myogenic murine C2C12 cells. Overall, TCF-ALP offers promise as an effective fluorescent/colorimetric probe for evaluating phosphatase activity in clinical assays or live cell systems.

Fluorescence assay for alkaline phosphatase based on ATP hydrolysis-triggered dissociation of cerium coordination polymer nanoparticles

Alkaline phosphatase (ALP) is a significant biomarker for diagnostics. Simple, selective and sensitive detection of ALP activity is thus of critical importance. In this study, an artful fluorescence assay for ALP is proposed based on adenosine triphosphate (ATP) hydrolysis-triggered disassociation and fluorescence quenching of cerium coordination polymer nanoparticles (CPNs). ATP, a recognized natural substrate of phosphatase, can serve as a superb "antenna" to sensitize the luminescence of Ce3+ with the aid of tris(hydroxymethyl) aminomethane (Tris), forming Ce3+-ATP-Tris CPNs. In the presence of ALP, ATP will be catalytically converted into adenosine and inorganic orthophosphate, however neither of them can sensitize Ce3+ in alkaline media. As a result, the obtained CPNs are disassociated, inducing the quenching of the fluorescence. On this basis, a straightforward fluorescence assay for ALP activity is rationally developed. The fluorescence quenching efficiency shows a linear relationship for ALP within the activity range from 0.1 to 10 mU mL-1 with a detection limit of 0.09 mU mL-1 under the optimal experimental conditions. Moreover, this facile yet effective fluorescence method featured simplicity, cost-effectiveness, high sensitivity and high selectivity and can be successfully utilized for the quantitative detection of ALP in human serum samples.

Ratiometric fluorescent sensor for visual determination of copper ions and alkaline phosphatase based on carbon quantum dots and gold nanoclusters

In this work, a novel ratiometric fluorescent sensor, based on carbon dots (CDs) and gold nanoclusters (AuNCs), is developed for highly sensitive and selective visual colorimetric detection of Cu²⁺ and alkaline phosphatase (ALP). The ratiometric fluorescent sensor was synthesized by covalently linking 11-mercaptoundecanoic acid (11-MUA)-stabilized AuNCs to the surface of amino-functionalized CD/SiO₂ nanoparticles. The red fluorescence of the AuNCs can be quenched by Cu²⁺ owing to coordination between Cu²⁺ and 11-MUA; however, the blue emission of the CDs was insensitive to Cu²⁺ owing to the protective silica shell. The quenching of the AuNCs' fluorescence returned when PPi was added because of the higher affinity between Cu²⁺ and PPi than that between Cu²⁺ and 11-MUA. In the presence of ALP, PPi was catalytically hydrolyzed into phosphate (Pi), which showed a much weaker affinity for Cu²⁺. Thus, Cu²⁺ ions were released, and the fluorescence of the AuNCs was quenched once more. Based on this principle, Cu²⁺ and ALP could be simultaneously detected. The developed ratiometric fluorescent sensor could detect Cu²⁺ over a range from 0.025 to 4 μM with a detection limit of 0.013 μM and ALP over a range from 0.12 to 15 U/L with a detection limit of 0.05 U/L. The present method was successfully applied for the detection of Cu²⁺ and ALP in real water samples and in human serum samples, respectively. This ratiometric fluorescent approach may provide a highly sensitive and accurate platform for visual Cu²⁺ and ALP sensing in environmental monitoring and medical diagnosis.

Photoelectrochemical determination of alkaline phosphatase activity based on a photo-excited electron transfer strategy

A sensitive photoelectrochemical (PEC) biosensor for determination of alkaline phosphatase (ALP) activity was constructed based on a photo-excited electron transfer strategy. Immobilization of CdTe quantum dots (QDs) on TiO₂ nanotube arrays (TNAs), addition of iron (III) and adenosine triphosphate (ATP) in turn can effectively adjust the photocurrent response of TNAs under visible light irradiation due to a photo-excited electron transfer process, and alkaline phosphatase (ALP) activity can be determined for its catalysis toward dephosphorylation of ATP. The preparation of CdTe QDs, construction of TNA/QD PEC biosensor and the mechanism of photo-excited electron transfer are investigated in the present work. Under the optimal experimental conditions, the TNA/QD PEC biosensor shows a low limits of detection (LODs) (0.05 U L^-1) and limits of quantification detection (LOQs) (0.15 U L^-1), wide linear range from 0.2 to 15 U L^-1, and good selectivity towards ALP determination, which has been successfully applied for human serum analysis with good precision (RSD ≤ 5.4%) and high accuracy (recovery rate, 91-112%).

Advanced Selection Methodologies for DNAzymes in Sensing and Healthcare Applications

DNAzymes have been widely explored owing to their excellent catalytic activity in a broad range of applications, notably in sensing and biomedical devices. These newly discovered applications have built high hopes for designing novel catalytic DNAzymes. However, the selection of efficient DNAzymes is a challenging process but one that is of crucial importance. Initially, systemic evolution of ligands by exponential enrichment (SELEX) was a labor-intensive and time-consuming process, but recent advances have accelerated the automated generation of DNAzyme molecules. This review summarizes recent advances in SELEX that improve the affinity and specificity of DNAzymes. The thriving generation of new DNAzymes is expected to open the door to several healthcare applications. Therefore, a significant portion of this review is dedicated to various biological applications of DNAzymes, such as sensing, therapeutics, and nanodevices. In addition, discussion is further extended to the barriers encountered for the real-life application of these DNAzymes to provide a foundation for future research.

Target-activated cascaded digestion amplification of exonuclease III aided signal-on and ultrasensitive fluorescence detection of ATP

In this work, a rapid, one-step and ultrasensitive signal-on fluorescence sensing for the detection of adenosine triphosphate (ATP) based on target-activated cascaded digestion amplification with Exo III aid was developed.

Facile Synthesis of Fluorescent Conjugated Polyelectrolytes Using Polydentate Sulfonate as Highly Selective and Sensitive Copper(II) Sensors

Fluorescent conjugated polyelectrolytes represent an exciting area of research into new chemosensors. By virtue of their rapid electron and energy transfer paths, these highly correlated, one-dimensional systems have been depicted as "molecular wires" and show "million-fold" sensitivity compared to monomolecular sensor analogs. In this paper, a novel polyelectrolyte sensor, the ttp-PPESO₃, has been designed by incorporating terpyridine and sulfonate functional groups into the polyelectrolyte. This specifically tailored sensor has displayed remarkable quenching response toward copper(II) with a detection limit of 14.7 nM (0.93 ppb). It is capable of selectively screening copper without interference from 12 common cations. Molecular modeling suggests that binding occurs through a coordination interaction of the terpyridine and sulfonate. The additional multidentate nature from the sulfonate offers extraordinary chelating ability to the analyte. We anticipate that this unique binding mode will provide insight for the design of future more sensitive and selective systems.

Nitrogen-doped Carbon Dots Mediated Fluorescent on-off Assay for Rapid and Highly Sensitive Pyrophosphate and Alkaline Phosphatase Detection

In this report, a novel fluorescent sensing platform using nitrogen-doped carbon dots (N-CDs) as probes for fluorescence signal transmission has been designed for the detection of significant biomolecules pyrophosphate (PPi) and alkaline phosphatase (ALP). The high fluorescent N-CDs could be selectively quenched by Cu2+, and recovered by the addition of PPi because PPi preferentially binds to Cu2+. Once ALP was introduced into the system, ALP can specifically hydrolyze PPi into Pi, the intense fluorescence of N-CDs could be quenched again due to the recombination of the as-released Cu2+ with N-CDs. So, fluorescence of N-CDs is regulated by an ALP-triggered reaction. Based on this strategy, we demonstrated that N-CDs could serve as a very effective fluorescent sensing platform for label-free, sensitive and selective detection of PPi and ALP with low detection limit of 0.16 μM and 0.4 U/L for PPi and ALP, respectively. Moreover, the assay time is just around 0.5 min for PPi and 30 min for ALP. This developed strategy shows remarkable advantages including sensitive, rapid, simple, convenient, and low-cost and so forth. Furthermore, this method was also successfully applied to monitor ALP in human serum, which indicates its great potential for practical applications in biological and clinical diagnosis.

Detection and quantification of ATP in human blood serum

Two fluorometric sensors based on the tri-serine tri-lactone scaffold and thiourea or sulfonamide moieties serving as hydrogen bond donors allowing for anion binding are described. The sensor utilizing thiourea as a recognition moiety shows fluorescence enhancement while the sensor with sulfonamide shows quenching upon addition of phosphates. Sensor arrays composed of two sensors are able to discriminate structurally similar organic phosphates in the presence of interferents in human blood serum. The quantitative analysis of ATP in human blood serum shows high accuracy (the root mean square error of prediction, 1.65%) without requiring any sample pretreatment.

An anionic conjugated polymer as a multi-action sensor for the sensitive detection of Cu(2+) and PPi, real-time ALP assaying and cell imaging

A Cu(2+) ensemble polyfluorene derivative, poly[5,5'-(((9H-fluorene-9,9-diyl)bis(hexane-6,1-diyl))bis(oxy))diisophthalate] sodium salt (PFT), displays unprecedented selectivity for PPi (LOD = 2.26 ppb) in aqueous solution as well as in random urine samples at physiological pH vis-a-vis monitoring ALP activity. Furthermore, intracellular imaging of Cu(2+) and PPi in mouse macrophage (J774A.1) and human breast cancer cells (MDA-MB231) was achieved to confirm the viability of PFT in biological systems.

Stimulus Response of Au-NPs@GMP-Tb Core-Shell Nanoparticles: Toward Colorimetric and Fluorescent Dual-Mode Sensing of Alkaline Phosphatase Activity in Algal Blooms of a Freshwater Lake

In this study, we demonstrate a colorimetric and fluorescent dual-mode method for alkaline phosphatase activity (APA) sensing in freshwater lake with stimuli-responsive gold nanoparticles@terbium-guanosine monophosphate (Au-NPs@GMP-Tb) core-shell nanoparticles. Initially, the core-shell nanoparticles were fabricated based on Au-NPs decorated with a fluorescent GMP-Tb shell. Upon being excited at 290 nm, the as-formed Au-NPs@GMP-Tb core-shell nanoparticles emit green fluorescence, and the decorated GMP-Tb shell causes the aggregation of Au-NPs. However, the addition of ALP destroys GMP-Tb shell, resulting in the release of Au-NPs from the shell into the solvent. As a consequence, the aggregated Au-NPs solubilizes with the changes in the UV-vis spectrum of the dispersion, and in the meantime, the fluorescence of GMP-Tb shell turns off, which constitutes a new mechanism for colorimetric and fluorescent dual-mode sensing of APA. With the method developed here, we could monitor the dynamic change of APA during an algal bloom of a freshwater lake, both by the naked eye and further confirmed by fluorometric determination. This study not only offers a new method for on-site visible detection of APA but also provides a strategy for dual-mode sensing mechanisms by the rational design of the excellent optical properties of Au-NPs and the adaptive inclusion properties of the luminescent infinite coordination polymers.

A self-directed and reconstructible immobilization strategy: DNA directed immobilization of alkaline phosphatase for enzyme inhibition assays

A DNA-directed immobilization technique is used to develop a common method for the reversible and self-directed immobilization of enzymes.

Carbon quantum dots-based recyclable real-time fluorescence assay for alkaline phosphatase with adenosine triphosphate as substrate

A convenient, reliable, and highly sensitive real-time assay for alkaline phosphatase (ALP) activity in the continuous and recyclable way is established on the basis of aggregation and disaggregation of carbon quantum dots (CQDs) through the competitive assay approach. CQDs and adenosine triphosphate (ATP) were used as the fluorescent indicator and substrate for ALP activity assessment, respectively. Richness of carboxyl groups on the surface of CQDs enables their severe aggregation triggered by cerium ions, which results in effective fluorescence quenching. Under the catalytic hydrolysis of ALP, ATP can be rapidly transformed to phosphate ions. Stronger affinity of phosphate ions to cerium ions than carboxyl groups is taken advantage of to achieve fluorescence recovery induced by redispersion of CQDs in the presence of ALP and ATP. Quantitative evaluation of ALP activity in a broad range from 4.6 to 383.3 U/L with the detection limit of 1.4 U/L can be realized in this way, which endows the assay with high enough sensitivity for practical detection in human serum. The assay can be used in a recyclable way for more than three times since the generated product CePO4 as a precipitate can be easily removed from the standard assay system. This strategy broadens the sensing application of fluorescent CQDs with excellent biocompatibility and provides an example based on disaggregation in optical probe development.

Poly(ferulic acid-co-tyrosine): Effect of the Regiochemistry on the Photophysical and Physical Properties en Route to Biomedical Applications

The photophysical and mechanical properties of novel poly(carbonate-amide)s derived from two biorenewable resources, ferulic acid (FA) and l-tyrosine ethyl ester, were evaluated in detail. From these two bio-based precursors, a series of four monomers were generated (having amide and/or carbonate coupling units with remaining functionalities to allow for carbonate formation) and transformed to a series of four poly(carbonate-amide)s. The simplest monomer, which was biphenolic and was obtained in a single amidation synthetic step, displayed bright, visible fluorescence that was twice brighter than FA. Multidimensional fluorescence spectroscopy of the polymers in solution highlighted the strong influence that regioselectivity and the degree of polymerization have on their photophysical properties. The regiochemistry of the system had little effect on the wettability, surface free energy, and Young’s modulus (ca. 2.5 GPa) in the solid state. Confocal imaging of solvent-cast films of each polymer revealed microscopically flat surfaces with fluorescent emission deep into the visible region. Fortuitously, one of the two regiorandom polymers (obtainable from the biphenolic monomer in only an overall two synthetic steps from FA and l-tyrosine ethyl ester) displayed the most promising fluorescent properties both in the solid state and in solution, allowing for the possibility of translating this system as a self-reporting or imaging agent in future applications. To further evaluate the potential of this polymer as a biodegradable material, hydrolytic degradation studies at different pH values and temperatures were investigated. Additionally, the antioxidant properties of the degradation products of this polymer were compared with its biphenolic monomer and FA.