A highly selective lead sensor based on a classic lead DNAzyme

Theranostic DNAzymes

DNAzymes are catalytically active DNA molecules that are obtained via in vitro selection. RNA-cleaving DNAzymes have attracted significant attention for both therapeutic and diagnostic applications due to their excellent programmability, stability, and activity. They can be designed to cleave a specific mRNA to down-regulate gene expression. At the same time, DNAzymes can sense a broad range of analytes. By combining these two functions, theranostic DNAzymes are obtained. This review summarizes the progress of DNAzyme for theranostic applications. First, in vitro selection of DNAzymes is briefly introduced, and some representative DNAzymes related to biological applications are summarized. Then, the applications of DNAzyme for RNA cleaving are reviewed. DNAzymes have been used to cleave RNA for treating various diseases, such as viral infection, cancer, inflammation and atherosclerosis. Several formulations have entered clinical trials. Next, the use of DNAzymes for detecting metal ions, small molecules and nucleic acids related to disease diagnosis is summarized. Finally, the theranostic applications of DNAzyme are reviewed. The challenges to be addressed include poor DNAzyme activity under biological conditions, mRNA accessibility, delivery, and quantification of gene expression. Possible solutions to overcome these challenges are discussed, and future directions of the field are speculated.

A Sensitive and Label-Free Pb(II) Fluorescence Sensor Based on a DNAzyme Controlled G-Quadruplex/Thioflavin T Conformation

Pb(II) can cause serious damaging effects to human health, and thus, the study of Pb²⁺ detection methods to sensitively and selectively monitor Pb(II) pollution has significant importance. In this work, we have developed a label-free fluorescence sensing strategy based on a Pb(II) DNAzyme cleavage and the ThT/G-quadruplex complex. In the presence of Pb(II), a G-rich tail was cut and released from the substrate strand, which then would form a G-quadruplex structure by combination with ThT dye. The fluorescence signal increase was then measured for sensitive Pb(II) quantification with a limit of detection of 0.06 nM. Our sensor also demonstrated high selectivity against six different metal ions, which is very important for the analysis of complex samples.

Designing a Biostable L-DNAzyme for Lead(II) Ion Detection in Practical Samples

A promising biosensor for effectively lead (II) ion detection in practical applications was developed by constructing a Pb2+-specific L-DNAzyme, the enantiomer of the natural nucleic acid-constructed D-DNAzyme. This fluorescent sensor contains the L-enzyme strand with a quencher at the 3' end, and the L-substrate strand with a fluorophore at the 5' and a quencher at the 3' ends that formed a complex. In the presence of Pb2+, the L-substrate is cut into two fragments, leading to the recovery of fluorescence. The sensor shows high sensitivity and selectivity for Pb2+ detection with a linear response in the range of 5-100 nM and a detection limit of 3 nM in aqueous solution. Importantly, based on that L-DNAzyme consists of non-natural nucleic acids, which is insensitive to nuclease digestion, protein adsorption and D-DNA hybridization, our sensor shows specific response to Pb2+ in practical water and serum samples. Therefore, it is expected that our L-DNAzyme-based strategy may offer a new method for developing simple, rapid and sensitive sensors in complex systems.

Fluorescent nanoprobes for sensing and imaging of metal ions: recent advances and future perspectives

Recent advances in nanoscale science and technology have generated nanomaterials with unique optical properties. Over the past decade, numerous fluorescent nanoprobes have been developed for highly sensitive and selective sensing and imaging of metal ions, both in vitro and in vivo. In this review, we provide an overview of the recent development of the design and optical properties of the different classes of fluorescent nanoprobes based on noble metal nanomaterials, upconversion nanoparticles, semiconductor quantum dots, and carbon-based nanomaterials. We further detail their application in the detection and quantification of metal ions for environmental monitoring, food safety, medical diagnostics, as well as their use in biomedical imaging in living cells and animals.

Biochemical sensing by nanofluidic crystal in a confined space

Electrokinetics at nanoscale has attracted broad attention as a promising conductivity based biochemical sensing principle with a good selectivity. The nanoparticle crystal, formed by self-assembling nanoparticles inside a microstructure, has been utilized to fulfill a nanoscale electrokinetics based biochemical sensing platform, named nanofluidic crystal in our previous works. This paper introduces a novel nanofluidic crystal scheme by packing nanoparticles inside a well-designed confined space to improve the device-to-device readout consistency. A pair of electrodes was patterned at the bottom of this tunnel-shaped confined space for ionic current recording. The readout from different chips (n = 16) varied within 8.4% under the same conditions, which guaranteed a self-calibration-free biochemical sensing. Biotin and Pb(2+) were successfully detected by using nanofluidic crystal devices packed with streptavidin and DNAzyme modified nanoparticles, respectively. The limits of detection (LODs) were both 1 nM. This electrically readable nanofluidic crystal sensing approach may find applications in low cost and fast disease screening in limited resource environments.

Biochemical and Biophysical Understanding of Metal Ion Selectivity of DNAzymes

This review summarizes research into the metal-binding properties of catalytic DNAzymes, towards the goal of understanding the structural properties leading to metal ion specificity. Progress made and insight gained from a range of biochemical and biophysical techniques are covered, and promising directions for future investigations are discussed.

An Ultrasensitive Light-up Cu(2+) Biosensor Using a New DNAzyme Cleaving a Phosphorothioate-Modified Substrate

Cu(2+) is a very important metal ion in biology, environmental science, and industry. Developing biosensors for Cu(2+) is a key topic in analytical chemistry. DNAzyme-based sensors are highly attractive for their excellent sensitivity, stability, and programmability. In the past decade, a few Cu(2+) biosensors were reported using DNAzymes with DNA cleavage or DNA ligation activity. However, they require unstable ascorbate or imidazole activation. So far, no RNA-cleaving DNAzymes specific for Cu(2+) are known. In this work, a phosphorothioate (PS) RNA-containing library was used for in vitro selection, and a few new Cu(2+)-specific RNA-cleaving DNAzymes were isolated. Among them, a DNAzyme named PSCu10 was studied further. It has only eight nucleotides in the enzyme loop with a cleavage rate of 0.1 min(-1) in the presence of 1 μM Cu(2+) at pH 6.0 (its optimal pH). Between the two diastereomers of the PS RNA chiral center, the R(p) isomer is 37 times more active than the S(p) one. Among the other divalent metal ions, only Hg(2+) can cleave the substrate due to its extremely high thiophilicity. A catalytic beacon sensor was designed with a detection limit of 1.6 nM Cu(2+) and extremely high selectivity. PSCu10 is specific for Cu(2+), and it has no cleavage in the presence of ascorbate, which reduces Cu(2+) to Cu(+).

Spinach RNA aptamer detects lead(II) with high selectivity

Spinach RNA aptamer contains a G-quadruplex motif that serves as a platform for binding and fluorescence activation of a GFP-like fluorophore. Here we show that Pb(2+) induces formation of Spinach's G-quadruplex and activates fluorescence with high selectivity and sensitivity. This device establishes the first example of an RNA-based sensor that provides a simple and inexpensive tool for Pb(2+) detection.

Biostable L-DNAzyme for Sensing of Metal Ions in Biological Systems

DNAzymes, an important type of metal ion-dependent functional nucleic acid, are widely applied in bioanalysis and biomedicine. However, the use of DNAzymes in practical applications has been impeded by the intrinsic drawbacks of natural nucleic acids, such as interferences from nuclease digestion and protein binding, as well as undesired intermolecular interactions with other nucleic acids. On the basis of reciprocal chiral substrate specificity, the enantiomer of D-DNAzyme, L-DNAzyme, could initiate catalytic cleavage activity with the same achiral metal ion as a cofactor. Meanwhile, by using the advantage of nonbiological L-DNAzyme, which is not subject to the interferences of biological matrixes, as recognition units, a facile and stable L-DNAzyme sensor was proposed for sensing metal ions in complex biological samples and live cells.

A rhodamine based “turn-on” fluorescent probe for Pb(ii) and live cell imaging

A novel “turn-on” fluorescent chemosensor RDP-1 based on rhodamine tri methoxy benzaldehyde conjugate was synthesized, which showed high selectivity and sensitivity towards recognition of Pb²⁺ in aqueous media.

Searching for a DNAzyme Version of the Leadzyme

The leadzyme refers to a small ribozyme that cleaves a RNA substrate in the presence of Pb(2+). In an optimized form, the enzyme strand contains only two unpaired nucleotides. Most RNA-cleaving DNAzymes are much longer. Two classical Pb(2+)-dependent DNAzymes, 8-17 and GR5, both contain around 15 nucleotides in the enzyme loop. This is also the size of most RNA-cleaving DNAzymes that use other metal ions for their activity. Such large enzyme loops make spectroscopic characterization difficult and so far no high-resolution structural information is available for active DNAzymes. The goal of this work is to search for DNAzymes with smaller enzyme loops. A simple replacement of the ribonucleotides in the leadzyme by deoxyribonucleotides failed to produce an active enzyme. A Pb(2+)-dependent in vitro selection combined with deep sequencing was then performed. After sequence alignment and DNA folding, a new DNAzyme named PbE22 was identified, which contains only 5 nucleotides in the enzyme catalytic loop. The biochemical characteristics of PbE22 were compared with those of the leadzyme and the two classical Pb(2+)-dependent DNAzymes. The rate of PbE22 rises with increase in Pb(2+) concentration, being 1.7 h(-1) in the presence of 100 μM Pb(2+) and reaching 3.5 h(-1) at 500 µM Pb(2+). The log of PbE22 rate rises linearly in a pH-dependent fashion (20 µM Pb(2+)) with a slope of 0.74. In addition, many other abundant sequences in the final library were studied. These sequences are quite varied in length and nucleotide composition, but some contain a few conserved nucleotides consistent with the GR5 structure. Interestingly, some sequences are active with Pb(2+) but none of them were active with even 50 mM Mg(2+), which is reminiscent of the difference between the GR5 and 8-17 DNAzymes.

Electrochemical Sensor for Lead Cation Sensitized with a DNA Functionalized Porphyrinic Metal-Organic Framework

An efficient electrochemical sensor was presented for lead cation detection using a DNA functionalized iron-porphyrinic metal-organic framework (GR-5/(Fe-P)n-MOF) as a probe. The newly designed probe showed both the recognition behavior of GR-5 to Pb(2+) with high selectivity and the excellent mimic peroxidase performance of (Fe-P)n-MOF. In the presence of Pb(2+), GR-5 could be specifically cleaved at the ribonucleotide (rA) site, which produced the short (Fe-P)n-MOF-linked oligonucleotide fragment to hybridize with hairpin DNA immobilized on the surface of screen-printed carbon electrode (SPCE). Because of the mimic peroxidase property of (Fe-P)n-MOF, enzymatically amplified electrochemical signal was obtained to offer the sensitive detection of Pb(2+) ranging from 0.05 to 200 nM with a detection limit of 0.034 nM. In addition, benefiting from the Pb(2+)-dependent GR-5, the proposed assay could selectively detect Pb(2+) in the presence of other metal ions. The SPCE based electrochemical sensor along with the GR-5/(Fe-P)n-MOF probe exhibited the advantages of low-cost, simple fabrication, high sensitivity and selectivity, providing potential application of on-site and real-time Pb(2+) detection in complex media.

Desulfurization Activated Phosphorothioate DNAzyme for the Detection of Thallium

Thallium (Tl) is a highly toxic heavy metal situated between mercury and lead in the periodic table. While its neighbors have been thoroughly studied for DNA-based sensing, little is known about thallium detection. In this work, in vitro selection of RNA-cleaving DNAzymes is carried out using Tl(3+) as the target metal cofactor. Both normal DNA and phosphorothioate (PS)-modified DNA are tested for this purpose. While no Tl(3+)-dependent DNAzymes are obtained, a DNA oligonucleotide containing a single PS-modified RNA nucleotide is found to cleave by ∼7% by Tl(3+) at the RNA position. The remaining 93% are desulfurized. By hybridization of this PS-modified oligonucleotide with the Tm7 DNAzyme, the cleavage yield increases to ∼40% in the presence of Tl(3+) and Er(3+). Tm7 is an Er(3+)-dependent RNA-cleaving DNAzyme. It cleaves only the normal substrate but is completely inactive using the PS-modified substrate. Tl(3+) desulfurizes the PS substrate to the normal substrate to be cleaved by Tm7 and Er(3+). This system is engineered into a catalytic beacon for Tl(3+) with a detection limit of 1.5 nM, which is below its maximal contamination limit defined by the U.S. Environmental Protection Agency (10 nM).

DNAzyme-based biosensors and nanodevices

DNAzymes, screened through in vitro selection, have shown great promise as molecular tools in the design of biosensors and nanodevices. The catalytic activities of DNAzymes depend specifically on cofactors and show multiple enzymatic turnover properties, which make DNAzymes both versatile recognition elements and outstanding signal amplifiers. Combining nanomaterials with unique optical, magnetic and electronic properties, DNAzymes may yield novel fluorescent, colorimetric, surface-enhanced Raman scattering (SERS), electrochemical and chemiluminescent biosensors. Moreover, some DNAzymes have been utilized as functional components to perform arithmetic operations or as "walkers" to move along DNA tracks. DNAzymes can also function as promising therapeutics, when designed to complement target mRNAs or viral RNAs, and consequently lead to down-regulation of protein expression. This feature article focuses on the most significant achievements in using DNAzymes as recognition elements and signal amplifiers for biosensors, and highlights the applications of DNAzymes in logic gates, DNA walkers and nanotherapeutics.

Functional nucleic acid-based sensors for heavy metal ion assays

Heavy metal contaminants such as lead ions (Pb(2+)), mercury ions (Hg(2+)) and silver ions (Ag(+)) can cause significant harm to humans and generate enduring bioaccumulation in ecological systems. Even though a variety of methods have been developed for Pb(2+), Hg(2+) and Ag(+) assays, most of them are usually laborious and time-consuming with poor sensitivity. Due to their unique advantages of excellent catalytic properties and high affinity for heavy metal ions, functional nucleic acids such as DNAzymes and aptamers show great promise in the development of novel sensors for heavy metal ion assays. In this review, we summarize the development of functional nucleic acid-based sensors for the detection of Pb(2+), Hg(2+) and Ag(+), and especially focus on two categories including the direct assay and the amplification-based assay. We highlight the emerging trends in the development of sensitive and selective sensors for heavy metal ion assays as well.

Highly sensitive and selective detection of Pb2+ ions using a novel and simple DNAzyme-based quartz crystal microbalance with dissipation biosensor

A novel, label-free DNAzyme-based quartz crystal microbalance with dissipation monitoring (QCM-D) biosensor was developed for the highly sensitive and specific detection of Pb(2+) ions. To enhance the performance of the sensor, oligonucleotide-functionalized gold nanoparticles were used for both frequency and dissipation amplification. This sensor was developed by immobilizing Pb(2+)-specific DNAzymes onto the QCM-D sensor surface and allowing them to hybridize with substrate-functionalized AuNPs. The DNAzyme catalyzed the cleavage of the substrate in the presence of Pb(2+) ions, causing the cleaved substrate-functionalized AuNPs to be removed from the sensor surface. Thus, Pb(2+) ions can be determined on-line by monitoring the change in frequency and dissipation signals. The results revealed that the sensor showed a sensitive response to Pb(2+) ions with detection limits of 14 nM and 20 nM for frequency and dissipation, respectively. This QCM-D biosensor also exhibited excellent selectivity toward Pb(2+) ions in the presence of other divalent metal ions. In addition, the approach was able to detect Pb(2+) in tap water, demonstrating its great potential for monitoring drinking water quality. The proposed sensor system described here represents a new class of lead ion sensor. Its simple detection strategy makes it feasible for 'pollution-free' detection; thus, the approach could have applications in on-line water quality monitoring.

A general approach for rational design of fluorescent DNA aptazyme sensors based on target-induced unfolding of DNA hairpins

DNA aptazymes are allosteric DNAzymes activated by the targets of DNA aptamers. They take the advantages of both aptamers and DNAzymes, which can recognize specific targets with high selectivity and catalyze multiple-turnover reactions for signal amplification, respectively, and have shown their great promise in many analytical applications. So far, however, the available examples of DNA aptazyme sensors are still limited in utilizing only several DNAzymes and DNA aptamers, most likely due to the lack of a general and simple approach for rational design. Herein, we have developed such a general approach for designing fluorescent DNA aptazyme sensors. In this approach, aptamers and DNAzymes are connected at the ends to avoid any change in their original sequences, therefore enabling the general use of different aptamers and DNAzymes in the design. Upon activation of the aptazymes by the targets of interest, the rate of fluorescence enhancement via the cleavage of a dually labeled substrate by the active aptazymes is then monitored for target quantification. Two DNAzymes and two aptamers are used as examples for the design of three fluorescent aptazyme sensors, and they all show high selectivity and sensitivity for the detection of their targets. More DNA aptazyme sensors for a broader range of targets could be developed by this general approach as long as suitable DNAzymes and aptamers are used.

Label-free detection of Pb2+ based on aggregation-induced emission enhancement of Au-nanoclusters

The luminescence intensity of the glutathione capped Au-nanoclusters could be enhanced due to the formation of aggregates, and was employed for the visual detection of Pb²⁺.

Photocaged DNAzymes as a general method for sensing metal ions in living cells

DNAzymes, which are sequences of DNA with catalytic activity, have been demonstrated as a potential platform for sensing a wide range of metal ions. Despite their significant promise, cellular sensing using DNAzymes has however been difficult, mainly because of the "always-on" mode of first-generation DNAzyme sensors. To overcome this limitation, a photoactivatable (or photocaged) DNAzyme was designed and synthesized, and its application in sensing Zn(II) in living cells was demonstrated. In this design, the adenosine ribonucleotide at the scissile position of the 8-17 DNAzyme was replaced by 2'-O-nitrobenzyl adenosine, rendering the DNAzyme inactive and thus allowing its delivery into cells intact, protected from nonspecific degradation within cells. Irradiation at 365 nm restored DNAzyme activity, thus allowing the temporal control over the sensing activity of the DNAzyme for metal ions. The same strategy was also applied to the GR-5 DNAzyme for the detection of Pb(II), thus demonstrating the possible scope of the method.

DNA-only cascade: a universal tool for signal amplification, enhancing the detection of target analytes

Diagnostic tests performed in the field or at the site of patient care would benefit from using a combination of inexpensive, stable chemical reagents and simple instrumentation. Here, we have developed a universal "DNA-only Cascade" (DoC) to quantitatively detect target analytes with increased speed. The DoC utilizes quasi-circular structures consisting of temporarily inactivated deoxyribozymes (DNAzymes). The catalytic activity of the DNAzymes is restored in a universal manner in response to a broad range of environmental and biological targets. The present study demonstrates DNAzyme activation in the presence of metal ions (Pb(2+)), small molecules (deoxyadenosine triphosphate) and nucleic acids homologous to genes from Meningitis-causing bacteria. Furthermore, DoC efficiently discriminates nucleic acid targets differing by a single nucleotide. When detection of analytes is orchestrated by functional nucleic acids, the inclusion of DoC reagents substantially decreases time for detection and allows analyte quantification. The detection of nucleic acids using DoC was further characterized for its capability to be multiplexed and retain its functionality following long-term exposure to ambient temperatures and in a background of complex medium (human serum).

Functionalization of cationic poly(p-phenylene ethynylene) with dendritic polyethylene enables efficient DNAzyme delivery for imaging Pb2+ in living cells

We report here an effective Pb²⁺-dependent DNAzyme (8-17 DNAzyme) delivery system based on the water-soluble dendritic polyethylene-cationic poly(p-phenylene ethynylene) for successfully imaging Pb²⁺ in living cells. For utilizing the 8-17 DNAzyme and its unique ability to catalyze a phosphodiester bond cleavage reaction in the presence of Pb²⁺, the distinctive conjugated polymer-based polyvalent nanocarrier design manages to load and transport 8-17 DNAzyme across cell membranes, and to realize the fluorescence imaging of Pb²⁺ in living cells. As shown by the confocal microscopy and flow cytometry observations, the fluorescence of Cy5.5 is obviously activated under the conditions of incubation with Pb²⁺, compared with the absence of Pb²⁺. Taken together, the study demonstrates the combination of the molecular-wire effect with "dendrimer effects" on their effective DNAzyme delivery and their cellular imaging Pb²⁺.

Sensing parts-per-trillion Cd(2+), Hg(2+), and Pb(2+) collectively and individually using phosphorothioate DNAzymes

Cadmium, mercury, and lead are collectively banned by many countries and regions in electronic devices due to their extremely high toxicity. To date, no sensing method can detect them as a group and also individually with sufficient sensitivity and selectivity. An RNA-cleaving DNAzyme (Ce13d) was recently reported to be active with trivalent lanthanides, which are hard Lewis acids. In this work, phosphorothioate (PS) modifications were systematically made on Ce13d. A single PS modification at the substrate cleavage site shifts the activity from being dependent on lanthanides to soft thiophilic metals. By incorporating the PS modification to another DNAzyme, a sensor array was prepared to detect each metal. Individual sensors have excellent sensitivity (limit of detection = 4.8 nM Cd(2+), 2.0 nM Hg(2+), and 0.1 nM Pb(2+)). This study provides a new route to obtain metal-specific DNAzymes by atomic replacement and also offers important mechanistic insights into metal binding and DNAzyme catalysis.

A portable DNAzyme-based optical biosensor for highly sensitive and selective detection of lead (II) in water sample

A portable, rapid and cost-effective DNAzyme based sensor for lead ions detection in water samples has been developed using an optical fiber sensor platform. The presence of Pb(2+) cleaves the DNAzymes and releases the fluorescent labeled fragments, which further hybridize with the complementary strands immobilized on the optic fiber sensor surface. Subsequent fluorescent signals of the hybridized fluorescent labeled fragment provides quantitative information on the concentrations of Pb(2+) with a dynamic range from 2-75 nM with a detection limit of 1.03 nM (0.21 ng mL(-1)). The proposed sensor also shows good selectivity against other mono and divalent metal ions and thus holds great potential for the construction of general DNAzyme-based sensing platform for the monitoring of other heavy metal ions. The sensor can be regenerated with a 1% SDS solution (pH 1.9) over 100 times without significant deterioration of the sensor performance. This portable sensor system can be potentially applied for on-site real-time inexpensive and easy-to-use monitoring of Pb(2+) in environmental samples such as wastewater effluents or water bodies.

Two Pb2+-specific DNAzymes with opposite trends in split-site-dependent activity

By splitting the catalytic core of DNAzymes into two halves, two Pb(2+)-specific DNAzymes retain partial activity, while they show opposite trends of activity as a function of the split site, revealing important nucleotides for catalysis and metal binding.

DNA as sensors and imaging agents for metal ions

Increasing interest in detecting metal ions in many chemical and biomedical fields has created demands for developing sensors and imaging agents for metal ions with high sensitivity and selectivity. This review covers recent progress in DNA-based sensors and imaging agents for metal ions. Through both combinatorial selection and rational design, a number of metal-ion-dependent DNAzymes and metal-ion-binding DNA structures that can selectively recognize specific metal ions have been obtained. By attachment of these DNA molecules with signal reporters such as fluorophores, chromophores, electrochemical tags, and Raman tags, a number of DNA-based sensors for both diamagnetic and paramagnetic metal ions have been developed for fluorescent, colorimetric, electrochemical, and surface Raman detection. These sensors are highly sensitive (with a detection limit down to 11 ppt) and selective (with selectivity up to millions-fold) toward specific metal ions. In addition, through further development to simplify the operation, such as the use of "dipstick tests", portable fluorometers, computer-readable disks, and widely available glucose meters, these sensors have been applied for on-site and real-time environmental monitoring and point-of-care medical diagnostics. The use of these sensors for in situ cellular imaging has also been reported. The generality of the combinatorial selection to obtain DNAzymes for almost any metal ion in any oxidation state and the ease of modification of the DNA with different signal reporters make DNA an emerging and promising class of molecules for metal-ion sensing and imaging in many fields of applications.

Graphene quantum dots/L-cysteine coreactant electrochemiluminescence system and its application in sensing lead(II) ions

A new coreactant electrochemiluminescence (ECL) system including single-layer graphene quantum dots (GQDs) and L-cysteine (L-Cys) was found to be able to produce strong cathodic ECL signal. The ECL signal of GQD/L-Cys coreactant system was revealed to be mainly dependent on some key factors, including the oxidation of L-Cys, the presence of dissolved oxygen and the reduction of GQDs. Then, a possible ECL mechanism was proposed for the coreactant ECL system. Furthermore, the ECL signal of the GQD/L-Cys system was observed to be quenched by lead(II) ions (Pb(2+)). After optimization of some important experimental conditions, including concentrations of GQDs and L-Cys, potential scan rate, response time, and pH value, an ECL sensor was developed for the detection of Pb(2+). The new methodology can offer a rapid, reliable, and selective detection of Pb(2+) with a detection limit of 70 nM and a dynamic range from 100 nM to 10 μM.