DNA Tetrahedra as Functional Nanostructures: From Basic Principles to Applications

Self-assembled supramolecular DNA tetrahedra composed of programmed sequence-engineered complementary base-paired strands represent elusive nanostructures having key contributions to the development and diverse applications of DNA nanotechnology. By appropriate engineering of the strands, DNA tetrahedra of tuneable sizes and chemical functionalities were designed. Programmed functionalities for diverse applications were integrated into tetrahedra structures including sequence-specific recognition strands (aptamers), catalytic DNAzymes, nanoparticles, proteins, or fluorophore. The article presents a comprehensive review addressing methods to assemble and characterize the DNA tetrahedra nanostructures, and diverse applications of DNA tetrahedra framework are discussed. Topics being addressed include the application of structurally functionalized DNA tetrahedra nanostructure for the assembly of diverse optical or electrochemical sensing platforms and functionalized intracellular sensing and imaging modules. In addition, the triggered reconfiguration of DNA tetrahedra nanostructures and dynamic networks and circuits emulating biological transformations are introduced. Moreover, the functionalization of DNA tetrahedra frameworks with nanoparticles provides building units for the assembly of optical devices and for the programmed crystallization of nanoparticle superlattices. Finally, diverse applications of DNA tetrahedra in the field of nanomedicine are addressed. These include the DNA tetrahedra-assisted permeation of nanocarriers into cells for imaging, controlled drug release, active chemodynamic/photodynamic treatment of target tissues, and regenerative medicine.

Topologically switchable and gated transcription machinery

Topological barriers control in nature the transcription machinery, thereby perturbing gene expression. Here we introduce synthetically designed DNA templates that include built-in topological barriers for switchable, triggered-controlled transcription of RNA aptamers. This is exemplified with the design of transcription templates that include reversible and switchable topological barriers consisting of a Sr2+-ion-stabilized G-quadruplex and its separation by kryptofix [2.2.2], KP, for the switchable transcription of the malachite green (MG) RNA aptamer, the T-A·T triplex barrier being separated by a fuel-strand for the cyclic triggered transcription of the 3,5-difluoro-4-hydroxybenzylidene imidazolinone (DFHBI)-binding aptamer, and the use of a photoactivated cis/trans azobenzene-modified nucleic acid barrier for the switchable "ON"/"OFF" transcription of the MG RNA aptamer. By applying a mixture of topologically triggered templates consisting of the photoresponsive barrier and the T-A·T triplex barrier, the gated transcription of the MG aptamer or the DFHBI-binding aptamer is demonstrated. In addition, a Sr2+-ion/KP topologically triggered DNA tetrahedra promoter-transcription scaffold, for the replication of the MG RNA aptamer, and T7 RNA polymerase are integrated into DNA-based carboxymethyl cellulose hydrogel microcapsules acting as cell-like assemblies. The switchable, reversible transcription of the MG RNA aptamer in a cell-like containment is introduced.

Dynamic Catalysis Guided by Nucleic Acid Networks and DNA Nanostructures

Nucleic acid networks conjugated to native enzymes and supramolecular DNA nanostructures modified with enzymes or DNAzymes act as functional reaction modules for guiding dynamic catalytic transformations. These systems are exemplified with the assembly of constitutional dynamic networks (CDNs) composed of nucleic acid-functionalized enzymes, as constituents, undergoing triggered structural reconfiguration, leading to dynamically switched biocatalytic cascades. By coupling two nucleic acid/enzyme networks, the intercommunicated feedback-driven dynamic biocatalytic operation of the system is demonstrated. In addition, the tailoring of a nucleic acid/enzyme reaction network driving a dissipative, transient, biocatalytic cascade is introduced as a model system for out-of-equilibrium dynamically modulated biocatalytic transformation in nature. Also, supramolecular nucleic acid machines or DNA nanostructures, modified with DNAzyme or enzyme constituents, act as functional reaction modules driving temporal dynamic catalysis. The design of dynamic supramolecular machines is exemplified with the introduction of an interlocked two-ring catenane device that is dynamically reversibly switched between two states operating two different DNAzymes, and with the tailoring of a DNA-tweezers device functionalized with enzyme/DNAzyme constituents that guides the dynamic ON/OFF operation of a biocatalytic cascade by opening and closing the molecular device. In addition, DNA origami nanostructures provide functional scaffolds for the programmed positioning of enzymes or DNAzyme for the switchable operation of catalytic transformations. This is introduced by the tailored functionalization of the edges of origami tiles with nucleic acids guiding the switchable formation of DNAzyme catalysts through the dimerization/separation of the tiles. In addition, the programmed deposition of two-enzyme/cofactor constituents on the origami raft allowed the dynamic photochemical activation of the cofactor-mediated biocatalytic cascade on the spatially biocatalytic assembly on the scaffold. Furthermore, photoinduced "mechanical" switchable and reversible unlocking and closing of nanoholes in the origami frameworks allow the "ON" and "OFF" operation of DNAzyme units in the nanoholes, confined environments. The future challenges and potential applications of dynamic nucleic acid/enzyme and DNAzyme conjugates are discussed in the conclusion paragraph.

Development of QDs-based nanosensors for heavy metal detection: A review on transducer principles and in-situ detection

Heavy metal pollution has severe threats to the ecological environment and human health. Thus, it is urgent to achieve the rapid, selective, sensitive and portable detection of heavy metal ions. To overcome the defects of traditional methods such as time-consuming, low sensitivity, high cost and complicated operation, QDs (Quantum dots)-based nanomaterials have been used in sensors to significantly improve the sensing performance. Due to their excellent physicochemical properties, high specific surface area, high adsorption and reactive capacity, nanomaterials could act as potential probes or offer enhanced sensitivity and create a promising nanosensors platform. In this review, the rapidly advancing types of QDs for heavy metal ions detection are first summarized. Modified with ligands, nanomaterials, or biomaterials, QDs are assembled on sensors by the interaction of electrostatic adsorption, chemical bonding, steric hindrance, and base-pairing. The stability of QDs-based nanosensors is improved by doping the elements to QDs, providing the reference substance, optimizing the assemble strategies and so on. Then, according to transducer principles, the two most typical sensor categories based on QDs: optical and electrochemical sensors are highlighted to be discussed. In the meanwhile, portable devices combining with QDs to adapt the practical detection in complex situations are summarized. The deficiencies and future challenges of QDs in toxicity, specificity, portability, multi-metal co-detection and degradation during the detection are also pointed out. In the end, the development trends of QDs-based nanosensors for heavy metal ions detection are discussed. This review presents an overall understanding, recent advances, current challenges and future outlook of QDs-based nanosensors for heavy metal detection.

Crystallographic Structure of Novel Types of AgI -Mediated Base Pairs in Non-canonical DNA Duplex Containing 2'-O,4'-C-Methylene Bridged Nucleic Acids

Metal-mediated base pairs have widespread applications, such as in DNA-metal nanodevices and sensors. Here, we focused on their sugar conformation in duplexes and observed the crystallographic structure of the non-canonical DNA/DNA duplex containing 2'-O,4'-C-methylene bridged nucleic acid in the presence of Ag^I ions. The X-ray crystallographic structure was successfully obtained at a resolution of 1.5 Å. A novel type of Ag^I -mediated base pair between the N1 positions of anti-conformation of adenines in the duplex was observed. In the central non-canonical region, a hexad nucleobase structure containing Ag^I -mediated base pairs between the N7 positions of guanines was formed. A highly bent non-canonical structure was formed at the origin of Ag^I -mediated base pairs in the central region. The bent duplex structure induced by the addition of Ag^I ions might become a powerful tool for dynamic structural changes in DNA nanotechnology applications.

Multiplexed and amplified chemiluminescence resonance energy transfer (CRET) detection of genes and microRNAs using dye-loaded hemin/G-quadruplex-modified UiO-66 metal-organic framework nanoparticles

Dye-loaded UiO-66 metal-organic framework nanoparticles (NMOFs) modified with catalytic hemin/G-quadruplex DNAzyme labels act as functional hybrid modules for the chemiluminescence resonance energy transfer (CRET) analysis of miRNAs (miRNA-155 or miRNA-21) or genes (p53 or BRCA1). The dye-loaded NMOFs (dye = fluorescein (Fl) or rhodamine 6G (Rh 6G)) are modified with hairpin probes that are engineered to include in their loop domains recognition sequences for the miRNAs or genes, and in their stem regions caged G-quadruplex domains. In the presence of the analytes miRNAs or genes, the hairpin structures are opened, leading, in the presence of hemin, to the self-assembly of hemin/G-quadruplex DNAzyme labels linked to the dye-loaded NMOFs. In the presence of luminol and H2O2, the hemin/G-quadruplex DNAzyme labels catalyze the generation of chemiluminescence that provides radiative energy to stimulate the process of CRET to the dye loaded in the NMOFs, resulting in the luminescence of the loaded dye without external excitation. The resulting CRET signals relate to the concentrations of the miRNAs or the genes and allow the sensitive analysis of miRNAs and genes. In addition, the DNA hairpin-functionalized dye-loaded NMOF sensing modules were further applied to develop amplified miRNA or gene CRET-based sensing platforms. The dye-loaded NMOFs were modified with hairpin probes that include in their loop domain the recognition sequences for miRNA-155 or miRNA-21 or the recognition sequences for the p53 or BRCA1 genes. Subjecting the hairpin-modified NMOFs to the respective miRNAs or genes, in the presence of two hairpins H i and H j that include in their stem regions caged G-quadruplex subunit domains, results in the analyte-triggered opening of the probe hairpin linked to the NMOFs, and the opened hairpin tethers induce the cross-opening of the hairpins H i and H j by the hybridization chain reaction, HCR, resulting in the assembly of G-quadruplex wires tethered to the NMOFs. The binding of hemin to the HCR-generated chains yields hemin/G-quadruplex DNAzyme wires that enhance, in the presence of luminol/H2O2, the CRET processes in the hybrid nanostructures. These amplification platforms lead to the amplified sensing of miRNAs and genes. By mixing the Fl- and Rh 6G-loaded hairpin-functionalized UiO NMOFs, the multiplexed CRET detection of miRNA-155, miRNA-21 and the p53 and BRCA1 genes is demonstrated.

Enzymatic Construction of Artificial Base Pairs: The Effect of Metal Shielding

Th formation of metal base pairs is a versatile method for the introduction of metal cations into nucleic acids that has been used in numerous applications including the construction of metal nanowires, development of energy, charge-transfer devices and expansion of the genetic alphabet. As an alternative, enzymatic construction of metal base pairs is an alluring strategy that grants access to longer sequences and offers the possibility of using such unnatural base pairs (UBPs) in SELEX experiments for the identification of functional nucleic acids. This method remains rather underexplored, and a better understanding of the key parameters in the design of efficient nucleotides is required. We have investigated the effect of methylation of the imidazole nucleoside (dIm^nMe TP) on the efficiency of the enzymatic construction of metal base pairs. The presence of methyl substituents on dImTP facilitates the polymerase-driven formation of dIm^4Me -Ag^I -dIm and dIm^2Me TP-Cr^III -dIm base pairs. Steric factors rather than the basicity of the imidazole nucleobase appear to govern the enzymatic formation of such metal base pairs. We also demonstrate the compatibility of other metal cations rarely considered in the construction of artificial metal bases by enzymatic DNA synthesis under both primer extension reaction and PCR conditions. These findings open up new directions for the design of nucleotide analogues for the development of metal base pairs.

Stimuli-Responsive Biomolecule-Based Hydrogels and Their Applications

This Review presents polysaccharides, oligosaccharides, nucleic acids, peptides, and proteins as functional stimuli-responsive polymer scaffolds that yield hydrogels with controlled stiffness. Different physical or chemical triggers can be used to structurally reconfigure the crosslinking units and control the stiffness of the hydrogels. The integration of stimuli-responsive supramolecular complexes and stimuli-responsive biomolecular units as crosslinkers leads to hybrid hydrogels undergoing reversible triggered transitions across different stiffness states. Different applications of stimuli-responsive biomolecule-based hydrogels are discussed. The assembly of stimuli-responsive biomolecule-based hydrogel films on surfaces and their applications are discussed. The coating of drug-loaded nanoparticles with stimuli-responsive hydrogels for controlled drug release is also presented.

Aqueous Phase Sensing of Fe3+ and Ascorbic Acid by a Metal-Organic Framework and Its Implication in the Construction of Multiple Logic Gates

A new Hf^IV -based metal-organic framework with UiO-66 topology was synthesized via a one-step solvothermal method by using 3-methyl-4-phenylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H₂ MPTDC) as a ligand. The MOF material showed a high stability in a broad pH range (from pH 2 to pH 12) in an aqueous medium. The presence of hydrophobic methyl and phenyl substituents in the carboxylic acid ligand and strong Hf-O bond play crucial roles in its stability. The new MOF material was systematically characterized by various techniques such as XRPD, N₂ sorption, thermogravimetric analyses and FT-IR spectroscopy. The photophysical properties of the MOF material were also examined by steady-state and time-resolved fluorescence studies. It was observed that the blue fluorescence of the MOF material was selectively quenched in the presence of Fe³⁺ ion in pure aqueous medium. A mechanistic study disclosed that quenching occurs via a strong inner filter effect (IFE) arising from Fe³⁺ ion in aqueous medium. Interestingly, the fluorescence of the MOF material can be recovered by elimination of the IFE of Fe³⁺ ion via reduction of Fe³⁺ ion by ascorbic acid (AA). Based on the fluorescence recovery by AA, a MOF based on-off-on probe was developed for the sensing of Fe³⁺ ion and AA in aqueous medium. Inspired by this reversible sensing event, we demonstrate basic (NOT, OR, YES, INHIBIT and IMP) and higher integrated logic operations utilizing this fluorescent MOF. This MOF-based logic systems could be potentially used for next-generation logic-gate based analytical applications as well as for the detection and discrimination of targeted molecules in various complex domains.

Ratiometric Fluorescent Nanoprobe for Highly Sensitive Determination of Mercury Ions

In this study, a novel dual-emission ratiometric fluorescent nanoprobe (RFN) was synthesized and ultilized for highly sensitive determination of mercury ions. In this nanoprobe, fluorescein isothiocyanate (FITC) doped silica (SiO₂) served as a reference signal, FITC–SiO₂ microspheres were synthesized and modified with amino groups, and then Au Nanoclusters (AuNCs) were combined with the amino groups on the surface of the FITC–SiO₂ microspheres to obtain the RFN. The selectivity, stability, and pH of the RFN were then optimized, and the determination of mercury ions was performed under optimal conditions. The probe fluorescence intensity ratio (F₅₂₀ nm/F₆₈₀ nm) and Hg²⁺ concentration (1.0 × 10⁻¹⁰ mol/L to 1.0 × 10⁻⁸ mol/L) showed a good linear relationship, with a correlation coefficient of R² = 0.98802 and a detection limit of 1.0 × 10⁻¹⁰ mol/L, respectively. The probe was used for the determination of trace mercury ion in water samples, and the recovery rate was 98.15~100.45%, suggesting a wide range of applications in monitoring pollutants, such as heavy metal ion and in the area of environmental protection.

From DNA Nanotechnology to Material Systems Engineering

In the past 35 years, DNA nanotechnology has grown to a highly innovative and vibrant field of research at the interface of chemistry, materials science, biotechnology, and nanotechnology. Herein, a short summary of the state of research in various subdisciplines of DNA nanotechnology, ranging from pure "structural DNA nanotechnology" over protein-DNA assemblies, nanoparticle-based DNA materials, and DNA polymers to DNA surface technology is given. The survey shows that these subdisciplines are growing ever closer together and suggests that this integration is essential in order to initiate the next phase of development. With the increasing implementation of machine-based approaches in microfluidics, robotics, and data-driven science, DNA-material systems will emerge that could be suitable for applications in sensor technology, photonics, as interfaces between technical systems and living organisms, or for biomimetic fabrication processes.

1,3,9-Triaza-2-oxophenoxazine: An Artificial Nucleobase Forming Highly Stable Self-Base Pairs with Three AgI Ions in a Duplex

Metal-mediated base pairs (MMBPs) formed by natural or artificial nucleobases have recently been developed. The metal ions can be aligned linearly in a duplex by MMBP formation. The development of a three- or more-metal-coordinated MMBPs has the potential to improve the conductivity and enable the design of metal ion architectures in a duplex. This study aimed to develop artificial self-bases coordinated by three linearly aligned Ag^I ions within an MMBP. Thus, artificial nucleic acids with a 1,3,9-triaza-2-oxophenoxazine (9-TAP) nucleobase were designed and synthesized. In a DNA/DNA duplex, self-base pairs of 9-TAP could form highly stable MMBPs with three Ag^I ions. Nine equivalents of Ag^I led to the formation of three consecutive 9-TAP self-base pairs with extremely high stability. The complex structures of 9-TAP MMBPs were determined by using electrospray ionization mass spectrometry and UV titration experiments. Highly stable self-9-TAP MMBPs with three Ag^I ions are expected to be applicable to new DNA nanotechnologies.

Nano-bio-computing lipid nanotablet

Using nanoparticles as substrates for computation enables algorithmic and autonomous controls of their unique and beneficial properties. However, scalable architecture for nanoparticle-based computing systems is lacking. Here, we report a platform for constructing nanoparticle logic gates and circuits at the single-particle level on a supported lipid bilayer. Our "lipid nanotablet" platform, inspired by cellular membranes that are exploited to compartmentalize and control signaling networks, uses a lipid bilayer as a chemical circuit board and nanoparticles as computational units. On a lipid nanotablet, a single-nanoparticle logic gate senses molecules in solution as inputs and triggers particle assembly or disassembly as an output. We demonstrate a set of Boolean logic operations, fan-in/fan-out of logic gates, and a combinational logic circuit such as a multiplexer. We envisage that our approach to modularly implement nanoparticle circuits on a lipid bilayer will create new paradigms and opportunities in molecular computing, nanoparticle circuits, and systems nanoscience.

Mix-&-Read Determination of Mercury(II) at Trace Levels with Hybrid Mesoporous Silica Materials Incorporating Fluorescent Probes by a Simple Mix-&-Load Technique

The synthesis, characterization, and application of mesoporous materials containing boron-dipyrromethene (BODIPY) moieties that allow the sensitive and selective detection of HgII in aqueous environments by fluorescence enhancement is reported. For this purpose, BODIPY dye I containing a thia-aza crown ether receptor as the fluorescent probe for the detection of HgII in aqueous environments is encapsulated into mesoporous materials to avoid self-quenching or aggregation in water. Determination of HgII is accomplished within a few seconds with high selectivity and sensitivity, reaching a limit of detection of 12 ppt. The determination of trace amounts of HgII in natural waters and in fish extracts is demonstrated by using our sensing material. The incorporation of the material into several μ-PAD strips yields a portable, cheap, quick, and easy-to-handle tool for trace HgII analysis in water.

Controlling the Catalytic and Optical Properties of Aggregated Nanoparticles or Semiconductor Quantum Dots Using DNA-Based Constitutional Dynamic Networks

Nucleic acid-based constitutional dynamic networks (CDNs) attract growing interest as a means to mimic complex biological networks. The triggered stabilization of the CDNs allows the dictated guided reversible reconfiguration and re-equilibration of the CDNs to other CDN configurations, where some of the constituents are up-regulated, while other constituents are down-regulated. Although substantial progress in controlling the adaptive dynamic control of the compositions of networks by means of auxiliary triggers was demonstrated, the use of CDNs as active ensembles for controlling chemical functionalities is still a challenge. We report on the assembly of signal-triggered CDN systems that guide the switchable aggregation of Au nanoparticles (NPs), thereby controlling their plasmonic properties and their catalytic functions (Au NPs-catalyzed oxidation of l-DOPA to dopachrome). In addition, we demonstrate that the triggered and orthogonal up-regulation and down-regulation of the constituents of the CDNs leads to the dictated aggregation of different-sized CdSe/ZnS quantum dots (QDs), cross-linked by K⁺-ion-stabilized G-quadruplex units. The incorporation of hemin into the G-quadruplex cross-linking units yields horseradish peroxidase-mimicking DNAzyme units that catalyze the generation of chemiluminescence via the oxidation of luminol by H₂O₂. The resulting chemiluminescence stimulates the chemiluminescence resonance energy transfer (CRET) process to the QDs, resulting in the luminescence of the two-sized QDs. By the application of appropriate triggers, the CDN-dictated up-regulation and down-regulation of the different-sized QDs aggregates are demonstrated, and the control over the photophysical functions of the different-sized QDs, by means of the CDNs, is highlighted.

2'-O,4'-C-Methylene-Bridged Nucleic Acids Stabilize Metal-Mediated Base Pairing in a DNA Duplex

The 2'-O,4'-C-methylene-bridged or locked nucleic acid (2',4'-BNA/LNA), with an N-type sugar conformation, effectively improves duplex-forming ability. 2',4'-BNA/LNA is widely used to improve gene knockdown in nucleic acid based therapies and is used in gene diagnosis. Metal-mediated base pairs (MMBPs), such as thymine (T)-Hg^II -T and cytosine (C)-Ag^I -C have been developed and used as attractive tools in DNA nanotechnology studies. This study aimed to investigate the application of 2',4'-BNA/LNA in the field of MMBPs. 2',4'-BNA/LNA with 5-methylcytosine stabilized the MMBP of C with Ag^I ions. Moreover, the 2',4'-BNA/LNA sugar significantly improved the duplex-forming ability of the DNA/DNA complex, relative to that by the unmodified sugar. These results suggest that the sugar conformation is important for improving the stability of duplex-containing MMBPs. The results indicate that 2',4'-BNA/LNA can be applied not only to nucleic acid based therapies, but also to MMBP technologies.

Mix-&-Read Determination of Mercury(II) at Trace Levels with Hybrid Mesoporous Silica Materials Incorporating Fluorescent Probes by a Simple Mix-&-Load Technique

The synthesis, characterization, and application of mesoporous materials containing boron-dipyrromethene (BODIPY) moieties that allow the sensitive and selective detection of HgII in aqueous environments by fluorescence enhancement is reported. For this purpose, BODIPY dye I containing a thia-aza crown ether receptor as the fluorescent probe for the detection of HgII in aqueous environments is encapsulated into mesoporous materials to avoid self-quenching or aggregation in water. Determination of HgII is accomplished within a few seconds with high selectivity and sensitivity, reaching a limit of detection of 12 ppt. The determination of trace amounts of HgII in natural waters and in fish extracts is demonstrated by using our sensing material. The incorporation of the material into several μ-PAD strips yields a portable, cheap, quick, and easy-to-handle tool for trace HgII analysis in water.

Structures, physicochemical properties, and applications of T–HgII–T, C–AgI–C, and other metallo-base-pairs

In this feature article, recent progress and future perspectives of metal-mediated base-pairs such as T–Hg(ii)–T and C–Ag(i)–C are presented.

G-quadruplex-based logic gates for HgII and AgI ions employing a luminescent iridium(iii) complex and extension of metal-mediated base pairs by polymerase

We report herein the synthesis of a series of cyclometallated iridium(iii) complexes as luminescent G-quadruplex-selective probes to construct AND, OR and INHIBIT logic gates for the detection of Hg^II and Ag^I ions.

Highly sensitive and selective spectroscopic detection of mercury(II) in water by using pyridylporphyrin-DNA conjugates

Single-labeled pyridylporphyrin-DNA conjugates are reported as highly sensitive and selective spectroscopic sensors for mercury(II) ions in water. The effects of chemical structure (thymine versus adenine), number of nucleotides (monomer versus octamer), and porphyrin metalation (Zn versus free base) on the sensitivity and selectivity of mercury(II) detection are explored. The results indicated that pyridylporphyrin rather than the nucleobase plays a crucial role in mercury(II) sensing, because porphyrin conjugates with both adenosine and thymidine exhibited excellent mercury(II) detection. Mercury(II) recognition was shown in emission quenching, as well as in a redshift of the porphyrin Soret band absorption. The limit of detection (LOD, 3σ/slope) of zinc(II) pyridylporphyrin-5'-oligodeoxythymidine (ZnPorT8) obtained by fluorescence quenching was calculated to be 21.14 nM. Other metal cations (Zn(2+), Cd(2+), Pb(2+), Mn(2+), Ca(2+), Ni(2+), Mg(2+), Fe(2+), Cu(2+), and Na(+)) did not interfere with the emission and absorption sensing of mercury(II). Free-base porphyrin-oligothymine conjugate 2HPorT8 displayed similar sensitivity to ZnPorT8 but different selectivity. The results also implied that the sensing properties of porphyrin-deoxythymidine conjugates could potentially be tuned by porphyrin metalation.

Detection of lead (II) with a "turn-on" fluorescent biosensor based on energy transfer from CdSe/ZnS quantum dots to graphene oxide

Graphene oxide (GO) sheets are mixed with the aptamer-functionalized CdSe/ZnS quantum dots (QDs). Consequently, the aptamer-conjugated QDs bind to the GO sheets to form a GO/aptamer-QD ensemble, which enables the energy transfer from the QDs to the GO sheets, quenching the fluorescence of QDs. The GO/aptamer-QD ensemble assay acts as a "turn-on" fluorescent sensor for Pb(2+) detection. When Pb(2+) ions are present in the assay, the interaction of Pb(2+) with the aptamer induces a conformational change in the aptamer, leading to the formation of a G-quadruplex/Pb(2+) complex. As a result, the QDs that are linked to the G-quadruplex/Pb(2+) complex are detached from the GO sheet, which "turns on" the fluorescence of the QDs. This sensor exhibits a limit of detection of 90pM and excellent selectivity toward Pb(2+) over a wide range of metal ions. The experiments have provided direct evidence that the fluorescence of QDs is quenched by GO via the nano-metal surface energy transfer (NSET) mechanism rather than the conventional Förster resonance energy transfer (FRET) process.