Sensitive detection of uric acid based on low-triggering-potential cathodic luminol electrochemiluminescence achieved by ReS2 nanosheets

The majority of previously reported cathodic electrochemiluminescence (ECL) systems often required very negative potential to be carried out, which has greatly limited their applications in the sensing field. Screening high-performance cathodic ECL systems with low triggering potential is a promising way to broaden their applications. In this work, rhenium disulfide nanosheets (ReS2 NS) have been revealed as an efficient co-promoter to realize low-triggering-potential cathodic luminol ECL. One strong cathodic ECL signal appeared at a potential of -0.3 V and one anodic ECL peak was obtained at -0.15 V under the reverse potential scan, which were caused by electrogenerated reactive oxygen species (ROS) from hydrogen peroxide. The generation of strong luminol ECL at low potential was the result of the electrocatalytic effect of ReS2 NS on the reduction of H2O2. The scavenging effect of uric acid (UA) on the ROS could significantly inhibit the cathodic ECL. As a result, an ECL sensor was proposed, which showed outstanding performance for the detection of UA in the range of 10 nM to 0.1 mM with a low detection limit of 1.53 nM. Moreover, the ECL sensor was successfully applied in the sensitive detection of UA in real samples. This work provides a new avenue to establish a low-potential cathodic ECL system, which will sufficiently expand the potential application of cathodic ECL in the sensing field.

Unraveling Mechanisms of Highly Efficient Yet Stable Electrochemiluminescence from Quantum Dots

With CdSe/CdS/ZnS core/shell/shell quantum dots (QDs) as the model system, time- and potential-resolved spectroelectrochemical measurements are successfully applied for studying the general mechanisms and kinetics of electrochemiluminescence (ECL) generation. The rate constant of electron injection from the cathode into a QD to form a negatively charged QD (QD-) increases monotonically from -0.88 V to -1.2 V (vs Ag/AgCl). Mainly due to the deep LUMO of the QDs, the resulting QD- as the key intermediate for ECL generation is structurally stable and possesses very slow spontaneous deionization channels. The latter (the main non-ECL channels) are usually 3-4 orders of magnitude slower than the rate constant of the successive hole injection from an active co-reactant into a QD-. The kinetic studies quantify the internal ECL quantum yield of ideal QD ECL emitters to be nearly identical to that of photoluminescence, which is near unity for the current system. Identification of the key intermediate, discovery of the related elementary steps, and determination of all rate constants not only establish a general framework for understanding ECL generation but also offer basic design rules for ECL emitters.

Advances in electrochemiluminescence luminophores based on small organic molecules for biosensing

Electrochemiluminescence (ECL) has several advantages, such as a near-zero background signal, high sensitivity, wide dynamic range, simplicity, and is widely used for sensing, imaging, and single cell analysis. ECL luminophores are the key factors in the performance of various applications. Among various luminophores, small organic luminophores exhibit many intriguing features including good biocompatibility, facile modification, well-defined molecular structure, and sustainable raw materials, making small organic luminophores attractive for the use in the ECL field. Although many great achievements have been made in the synthesis of new small organic luminophores, solving various challenges, and expanding new applications, there are almost no comprehensive reviews on small organic ECL luminophores. In this review, we briefly introduce the advantages and emission mechanisms of small organic ECL luminophores, summarize the main types, molecular characteristics, and ECL properties of most existing small organic ECL luminophores, and present the important applications and design principles in sensors, imaging, single cell analysis, sterilization, and other fields. Finally, the challenges and outlook of organic ECL luminophores to be popularized in biosensing applications are also discussed.

Monitoring leaching of Cd2+ from cadmium-based quantum dots by an Cd aptamer fluorescence sensor

Quantum Dots (QDs) have been demonstrated with outstanding optical properties and thus been widely used in many biological and biomedical studies. However, previous studies have shown that QDs can cause cell toxicity, mainly attributable to the leached Cd2+. Therefore, identifying the leaching kinetics is very important to understand QD biosafety and cytotoxicity. Toward this goal, instrumental analyses such as inductively coupled plasma mass spectrometry (ICP-MS) have been used, which are time-consuming, costly and do not provide real-time or spatial information. To overcome these limitations, we report herein a fast and cost-effective fluorescence sensor based a Cd2+-specific aptamer for real-time monitoring the rapid leaching kinetics of QDs in vitro and in living cells. The sensor shows high specificity towards Cd2+ and is able to measure the Cd2+ leached either from water-dispersed CdTe QDs or two-layered CdSe/CdS QDs. The sensor is then used to study the stability of these two types of QDs under conditions to mimic cellular pH and temperature and the results from the sensor are similar to those obtained from ICP-MS. Finally, the sensor is able to monitor the leaching of Cd2+ from QDs in HeLa cells. The fluorescence aptamer sensor described in this study may find many applications as a tool for understanding biosafety of numerous other Cd-based QDs, including leaching kinetics and toxicity mechanisms in living systems.

Surface passivation of intensely luminescent all-inorganic nanocrystals and their direct optical patterning

All-inorganic nanocrystals (NCs) are of great importance in a range of electronic devices. However, current all-inorganic NCs suffer from limitations in their optical properties, such as low fluorescence efficiencies. Here, we develop a general surface treatment strategy to obtain intensely luminescent all-inorganic NCs (ILANs) by using designed metal salts with noncoordinating anions that play a dual role in the surface treatment process: (i) removing the original organic ligands and (ii) binding to unpassivated Lewis basic sites to preserve the photoluminescent (PL) properties of the NCs. The absolute photoluminescence quantum yields (PLQYs) of red-emitting CdSe/ZnS NCs, green-emitting CdSe/CdZnSeS/ZnS NCs and blue-emitting CdZnS/ZnS NCs in polar solvents are 97%, 80% and 72%, respectively. Further study reveals that the passivated Lewis basic sites of ILANs by metal cations boost the efficiency of radiative recombination of electron-hole pairs. While the passivation of Lewis basic sites leads to a high PLQY of ILANs, the exposed Lewis acidic sites provide the possibility for in situ tuning of the functions of NCs, creating opportunities for direct optical patterning of functional NCs with high resolution.

Functionalization of 2D MoS2 Nanosheets with Various Metal and Metal Oxide Nanostructures: Their Properties and Application in Electrochemical Sensors

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained considerable attention due to their distinctive properties and broad range of possible applications. One of the most widely studied transition metal dichalcogenides is molybdenum disulfide (MoS₂). The 2D MoS₂ nanosheets have unique and complementary properties to those of graphene, rendering them ideal electrode materials that could potentially lead to significant benefits in many electrochemical applications. These properties include tunable bandgaps, large surface areas, relatively high electron mobilities, and good optical and catalytic characteristics. Although the use of 2D MoS₂ nanosheets offers several advantages and excellent properties, surface functionalization of 2D MoS₂ is a potential route for further enhancing their properties and adding extra functionalities to the surface of the fabricated sensor. The functionalization of the material with various metal and metal oxide nanostructures has a significant impact on its overall electrochemical performance, improving various sensing parameters, such as selectivity, sensitivity, and stability. In this review, different methods of preparing 2D-layered MoS₂ nanomaterials, followed by different surface functionalization methods of these nanomaterials, are explored and discussed. Finally, the structure-properties relationship and electrochemical sensor applications over the last ten years are discussed. Emphasis is placed on the performance of 2D MoS₂ with respect to the performance of electrochemical sensors, thereby giving new insights into this unique material and providing a foundation for researchers of different disciplines who are interested in advancing the development of MoS₂-based sensors.

Determination of β-amyloid oligomer using electrochemiluminescent aptasensor with signal enhancement by AuNP/MOF nanocomposite

In order to effectively and conveniently detect the β-amyloid oligomer (AβO) for earlier diagnosis of Alzheimer's disease (AD), a disposable aptamer biosensor has been developed with high performance, facile operation, and low cost. Using a nanocomposite by in situ reduction of chloroauric acid to decorate Au nanoparticles (AuNPs) on Fe-MIL-88NH₂ material via Au-N bond to effectively enhance the electrochemiluminescence (ECL) of luminol, the functioned basal electrode provides adequate background for sensing response. When the aptamer is linked via Au-S bond on the surface, the sensor gets the ability of specific recognition and coalescence toward the target (AβO). After incubating the sample on the aptasensor, its ECL signal is inhibited owing to the steric hindrance of the AβO macromolecules. The relative inhibition ratio linearly depends to the logarithm of AβO concentration in the range 0.1 pM to 10 pM, with an LOD of 71 fM. The aptasensor has high selectivity to AβO among its analogs. The recoveries in human serum were 98.9-105.4%. This research provides a new approach for sensitive detection of AβO in clinic laboratories for investigation and diagnosis of AD.

Solvent-assisted strongly enhanced light-emitting electrochemiluminescent devices for lighting applications

Rubrene-based electrochemiluminescence (r-ECL) cells with two different solvent systems is prepared, one in a co-solvent system with a mixture of 1,2-dichlorobenzene and propylene carbonate (DCB : PC, v/v 3 : 1) and another in a single solvent system of tetrahydrofuran (THF), as the medium to form a liquid-electrolyte (L-El). By simply changing the solvent systems, from the co-solvent DCB : PC (v/v 3 : 1) to the single solvent THF, with the same amount of electrochemiluminescent rubrene (5 mM) and Li-based salt, a dramatically enhanced brightness of over 30 cd m⁻² is observed for the r-ECL cell in L-El_THF which is approximately 7-times higher than the brightness of 5 cd m⁻² observed for the r-ECL in L-El_{DCB:PC(v/v 3:1)}.

Simple and easily preparable electrochemiluminescent device (ECLD) is a promising alternative to LED and/or OLED for lighting application.

Quantum Dots with Highly Efficient, Stable, and Multicolor Electrochemiluminescence

Outstanding photoluminescence (PL) and electroluminescence properties of quantum dots (QDs) promise possibilities for them to meet challenging expectations of electrochemiluminescence (ECL), which at present relies on inefficient and spectral-irresolvable emitters based on transition-metal complexes (such as Ru(bpy)3 2+). However, ECL is reported to be extremely sensitive to the surface traps on the QDs likely because of the spatially and temporally separated electrochemical charge injections. Results here reveal that, by engineering the interior inorganic structure (CdSe/CdS/ZnS core/shell/shell structure) and inorganic-organic interface using new synthetic methods, the trap-insensitive QDs with near-unity PL quantum yield and monoexponential PL decay dynamics in water generated narrow band-edge ECL with efficiencies about six orders of magnitude higher than that of the standard Ru(bpy)3 2+. The band-edge and spectrally resolved ECL from CdSe/CdS/ZnS core/shell/shell QDs demonstrated a new readout scheme using electrochemical potential. Excellent ECL performance of QDs uncovered here offer opportunities to realize the full potential of ECL for biomedical detection and diagnosis.

Electrochemiluminescence Immunosensor Based on Au Nanocluster and Hybridization Chain Reaction Signal Amplification for Ultrasensitive Detection of Cardiac Troponin I

Measurement of cardiac troponin I in the blood is crucial for the early diagnosis of acute myocardial infarction. Herein, a novel and ultrasensitive electrochemiluminescence (ECL) immunosensor has been developed for determination of cardiac troponin I (cTnI) by using Au nanoclusters and hybridization chain reaction (HCR) signal amplification. In this ECL immunosensor, Au nanoclusters were dual-labeled at each end of hairpin DNA (H₁ and H₂) and acted as the luminophore. DNA initiator strands (T₁) and secondary antibody (Ab₂) were conjugated on Au nanoparticles (AuNPs) to obtain a smart probe (Ab₂-AuNP-T₁). In the presence of target cTnI, the sandwiched immunocomplex composed of cTnI, Ab₁, and Ab₂-AuNP-T₁ was formed. Then the initiator strands T₁ of Ab₂-AuNP-T₁ opened the hairpin DNA structures and triggered a cascade of hybridization events. Consequently, a large number of Au NCs were indirectly modified on the surface of the electrode, which could react with the coreactant (K₂S₂O₈) and emit a strong ECL signal. Under the optimal conditions, the immunosensor exhibited a wide detection range for cTnI from 5 fg/mL to 50 ng/mL and a low detection limit of 1.01 fg/mL (S/N = 3). Because of the excellent specificity, stability, and reproducibility of the proposed ECL-HCR sensor, it has a great application prospect for cTnI detection in clinical diagnosis.

Highly sensitive bioaffinity electrochemiluminescence sensors: Recent advances and future directions

Electrogenerated chemiluminescence (also called electrochemiluminescence and abbreviated ECL) has attracted much attention in various fields of analysis due to the potential remarkably high sensitivity, extremely wide dynamic range and excellent controllability. Electrochemiluminescence biosensor, by taking the advantage of the selectivity of the biological recognition elements and the high sensitivity of ECL technique was applied as a powerful analytical device for ultrasensitive detection of biomolecule. In this review, we summarize the latest sensing applications of ECL bioanalysis in the field of bio affinity ECL sensors including aptasensors, immunoassays and DNA analysis, cytosensor, molecularly imprinted sensors, ECL resonance energy transfer and ratiometric biosensors and give future perspectives for new developments in ECL analytical technology. Furthermore, the results herein discussed would demonstrate that the use of nanomaterials with unique chemical and physical properties in the ECL biosensing systems is one of the most interesting research lines for the development of ultrasensitive electrochemiluminescence biosensors. In addition, ECL based sensing assays for clinical samples analysis and medical diagnostics and developing of immunosensors, aptasensors and cytosensor for this purpose is also highlighted.

A bifunctional nonenzymatic flexible glucose microsensor based on CoFe-Layered double hydroxide

A bifunctional flexible glucose microsensor has been successfully fabricated by directly growing a layered double hydroxide nanosheet array (LDH-NSA) on Ni wire. The as-obtained CoFe-LDH-NSA exhibits promising performances in electrochemical and colorimetric detection of glucose with high sensitivity and selectivity. This work demonstrates an effective strategy to fabricate multi-functional glucose nonenzyme sensors.

A novel electrochemiluminescence resonance energy transfer system for simultaneous determination of two acute myocardial infarction markers using versatile gold nanorods as energy acceptors

A spatial- and potential-resolved platform coupled with the ECL-RET strategy was developed for simultaneous dual-target detection.

Electrogenerated chemiluminescence of black phosphorus nanosheets and its application in the detection of H2O2

Black phosphorus nanosheets (BPNS) were synthesized from BP crystals through liquid exfoliation coupled with ultrasonic methods under aqueous conditions.

A single-electrode electrochemical system for multiplex electrochemiluminescence analysis based on a resistance induced potential difference

A single-electrode electrochemical system uses only one electrode for multiplex experiments, and is a highly cheap platform for high throughput analysis.

Developing low-cost and simple electrochemical systems is becoming increasingly important but still challenged for multiplex experiments. Here we report a single-electrode electrochemical system (SEES) using only one electrode not only for a single experiment but also for multiplex experiments based on a resistance induced potential difference. SEESs for a single experiment and multiplex experiments are fabricated by attaching a self-adhesive label with a hole and multiple holes onto an ITO electrode, respectively. This enables multiplex electrochemiluminescence analysis with high sensitivity at a very low safe voltage using a smartphone as a detector. For the multiplex analysis, the SEES using a single electrode is much simpler, cheaper and more user-friendly than conventional electrochemical systems and bipolar electrochemical systems using electrode arrays. Moreover, SEESs are free from the electrochemiluminescent background problem from driving electrodes in bipolar electrochemical systems. Since numerous electrodes and cover materials can be used to fabricate SEESs readily and electrochemistry is being extensively used, SEESs are very promising for broad applications, such as drug screening and high throughput analysis.

Efficient photoelectrochemical water splitting on ultrasmall defect-rich TaOx nanoclusters enhanced by size-selected Pt nanocluster promoters

Formation of nanoclusters has attracted a lot of attention in recent years because of their distinct properties from isolated atoms and bulk solids. Here, we focus on the catalytic properties of supported transition metal oxide nanoclusters, such as TaO₂, with a well-defined size distribution below 10 nm. We show that their catalytic performance can be greatly enhanced by introducing a reaction promoter such as Pt. Different combinations of precisely size-selected, defect-rich TaO_x and Pt nanoclusters are produced by a gas-phase aggregation technique in a special DC magnetron sputtering system. Argon flow rate and aggregation length are carefully optimized to control the sizes of these ultrasmall TaO_x and Pt nanoclusters by using a quadrupole mass filter, and TEM studies reveal the different crystalline nature of TaO_x (amorphous) and Pt (crystalline) nanoclusters. We have further demonstrated the size-dependent photoanode activity of (TaO_x, Pt) nanocluster systems in a photoelectrochemical water splitting reaction, where the Pt nanocluster promoters are found to provide a significant enhancement in the photocurrent density, approximately tripled that was observed from just TaO_x nanocluster catalysts alone. The photocurrent density and photoconversion efficiency tend to reduce when Pt nanoclusters become overpopulated due to blocking of the photosensitive TaO_x surface. Reducing the Pt nanocluster size resolves this problem by incorporating a greater number of smaller nanocluster promoters without blocking TaO_x, which leads to further enhancement in the photocurrent density. The enhanced photocatalytic activity is attributed to synergetic effects introduced by the Pt nanoclusters that act as temporary charge storage sites to facilitate effective separation of a large number of electron-hole pairs, generated from a large number of active sites on the defect-rich amorphous TaO_x nanoclusters upon illumination.

Highly sensitive electrochemiluminescence detection of a prostate cancer biomarker

Prostate-specific membrane antigen (PSMA), a glycoprotein expressed in the prostatic epithelium endowed with enzymatic activity, is a very promising diagnostic marker for the early detection of prostate cancer. In this study, we report a novel electrochemiluminescence ELISA-like immunosensor based on carbon nanotubes and a highly specific sandwich immunoassay for the PSMA detection. To fabricate the device, an optically transparent electrode was modified with doubly functionalized multi-walled carbon nanotubes carrying amine groups and a monoclonal anti-PSMA antibody. Subsequently, to complete the sandwich immunosensing device, a second specific monoclonal anti-PSMA antibody was labelled with a electrochemiluminescent probe. Under optimized experimental conditions, the proposed sensing device exhibits a performance exceeding that of the state of-the-art in terms of the limit of detection (LOD) and limit of quantification (LOQ) as good as 0.88 ng mL^-1 and 2.60 ng mL^-1, respectively, in real complex samples such as cell lysates. In addition, the unique role of carbon nanotubes is also discussed by comparison with an analogue sensor assembled without the nanocarbon-based material.

Polymorph-Dependent Electrogenerated Chemiluminescence of Low-Dimensional Organic Semiconductor Structures for Sensing

A sensitive electrogenerated chemiluminescence (ECL) sensor with an organic semiconductor as active material for detecting trace amounts of molecules has been highly desired. However, the crystal structure responses of the ECL properties of the organic semiconductor materials, that is, structure-property relationship, is not clear, which limits the development of the sensitive ECL sensors. Herein, for the first time, we reported a novel concept for molecular-stacking-arrangement-dependent electrogenerated chemiluminescence properties of organic semiconductor rubrene microstructures. The rubrene 1D microwires and 2D hexagonal plates with different polymorphs (triclinic and monoclinic) were controllably constructed with the reprecipitation method. The supersaturation of the rubrene molecules plays an important role in the thermodynamically and kinetically dominated process of growth, which affects not only the polymorphs but also the morphology of the obtained microstructures. These microstructures show good optoelectronic properties, which are used as active ECL materials for the construction of ECL sensors. The ECL sensors exhibited distinct electrogenerated chemiluminescence properties, probably related to different inherent crystal-structure-dependent triplet-triplet annihilation rate and charge-transfer rate. The sensors manifested electrogenerated chemiluminescence responses in broad linear range for the monitoring of creatinine molecules.

Facile heat reflux synthesis of blue luminescent carbon dots as optical nanoprobes for cellular imaging

Carbon dots with good temperature-sensitive photoluminescence are facilely synthesized and their luminescence properties and cellular imaging applications are investigated.

Cyclometalated iridium(III) chelates-a new exceptional class of the electrochemiluminescent luminophores

Recent development of the phosphorescent cyclometalated iridium(III) chelates has enabled, due to their advantageous electrochemical and photo-physical properties, important breakthroughs in many photonic applications. This particular class of 5d(6) ion complexes has attracted increasing interest because of their potential application in electroluminescence devices with a nearly 100 % internal quantum efficiency for the conversion of electric energy to photons. Similar to electroluminescence, the cyclometalated iridium(III) chelates have been successfully applied in the electricity-to-light conversion by means of the electrochemiluminescence (ECL) processes. The already reported ECL systems utilizing the title compounds exhibit extremely large ECL efficiencies that allow one to envisage many potential application for them, especially in further development of ECL-based analytical techniques. This review, based on recently published papers, focuses on the ECL properties of this very exciting class of organometallic luminophores. The reported work, describing results from fundamental as well as application-oriented investigations, will be surveyed and briefly discussed. Graphical abstract Depending on the chemical nature of the cyclometalated irdium(III) chelate different colours of the emitted light can be produced during electrochemical excitation.

Recent advances in electrochemiluminescence

The great success of electrochemiluminescence (ECL) for in vitro diagnosis (IVD) and its promising potential in light-emitting devices greatly promote recent ECL studies. More than 45% of ECL articles were published after 2010, and the first international meeting on ECL was held in Italy in 2014. This critical review discusses recent vibrant developments in ECL, and highlights novel ECL phenomena, such as wireless ECL devices, bipolar electrode-based ECL, light-emitting electrochemical swimmers, upconversion ECL, ECL resonance energy transfer, thermoresponsive ECL, ECL using shape-controlled nanocrystals, and ECL as an ion-selective electrode photonic reporter, a paper-based microchip, and a self-powered microfluidic ECL platform. We also comment on the latest progress in bioassays, light-emitting devices and, the computational approach for the ECL mechanism study. Finally, perspectives and key challenges in the near future are addressed (198 references).

Anodic electrogenerated chemiluminescence of self-assembled peptide nanotubes in an aqueous system

Anodic ECL of a self-assembled peptide nanotube modified electrode in an aqueous system was firstly observed using tri-n-propylamine (TPrA) as a coreactant.

Fabrication of fluorescent SiO2@zeolitic imidazolate framework-8 nanosensor for Cu2+ detection

The formation of core–shell SiO₂@ZIF-8 nanostructures for Cu²⁺ detection is reported here.

Anchoring AgBr nanoparticles on nitrogen-doped graphene for enhancement of electrochemiluminescence and radical stability

AgBr nanoparticles anchored nitrogen-doped graphene nanocomposites were designed to obtain enhanced electrochemiluminescence intensity and better stability.

Conjugated polymer dots/oxalate anodic electrochemiluminescence system and its application for detecting melamine

The anodic electrochemiluminescence behavior of an ammonolysis product of PFO in aqueous solution.

Ratiometric ECL of heterodinuclear Os–Ru dual-emission labels

The first ratiometric ECL of heterodinuclear Os–Ru dual-emission labels.

Size-selected TiO₂ nanocluster catalysts for efficient photoelectrochemical water splitting

Nanoclusters (NCs) are of great interest because they provide the link between the distinct behavior of atoms and nanoparticles and that of bulk materials. Here, we report precisely controlled deposition of size-selected TiO2 NCs produced by gas-phase aggregation in a special magnetron sputtering system. Carefully optimized aggregation length and Ar gas flow are used to control the size distribution, while a quadrupole mass filter provides precise in situ size selection (from 2 to 15 nm). Transmission electron microscopy studies reveal that NCs larger than a critical size (∼8 nm) have a crystalline core with an amorphous shell, while those smaller than the critical size are all amorphous. The TiO2 NCs so produced exhibit remarkable photoelectrochemical water splitting performance in spite of a small amount of material loading. NCs of three different sizes (4, 6, and 8 nm) deposited on H-terminated Si(100) substrates are tested for the photoelectrochemical catalytic performance, and significant enhancement in photocurrent density (0.8 mA/cm(2)) with decreasing NC size is observed with a low saturation voltage of -0.22 V vs Ag/AgCl (0.78 V vs RHE). The enhanced photoconductivity could be attributed to the increase in the specific surface area and increase in the number of active (defect) sites in the amorphous NCs. The unique advantages of the present technique will be further exploited to develop applications based on tunable, size-selected NCs.

Ruthenium polypyridine complexes combined with oligonucleotides for bioanalysis: a review

Ruthenium complexes are among the most interesting coordination complexes and they have attracted great attention over the past decades due to their appealing biological, catalytic, electronic and optical properties. Ruthenium complexes have found a unique niche in bioanalysis, as demonstrated by the substantial progress made in the field. In this review, the applications of ruthenium complexes coordinated with polypyridine ligands (and analogues) in bioanalysis are discussed. Three main detection methods based on electrochemistry, electrochemiluminescence, and photoluminscence are covered. The important targets, including DNA and other biologically important targets, are detected by specific biorecognition with the corresponding oligonucleotides as the biorecognition elements (i.e., DNA is probed by its complementary strand and other targets are detected by functional nucleic acids, respectively). Selected examples are provided and thoroughly discussed to highlight the substantial progress made so far. Finally, a brief summary with perspectives is included.