Aptamers are artificial oligonucleotide receptors originated from in vitro selection (SELEX). 1 In principle, aptamers with high specificity and affinity can be selected for any given target, ranging from small molecules to large proteins and even cells. 2 Therefore, aptamers are widely recognized as highly promising tools for a variety of important applications. 3,4 Aptamers are particularly useful as the biosensing element as they are chemically stable, readily available, and offer high flexibility in biosensor design. [5][6][7][8][9][10][11] Recently, Heeger, Plaxco, and others developed a series of novel electrochemical aptamer-based (E-AB) sensors for thrombin, cocaine, and potassium, [12][13][14] an analogous version to the electrochemical DNA (E-DNA) sensor. 8,15 These E-AB sensors are based on binding-induced conformational changes of redox-tagged and surface-confined aptamers, which have proven highly sensitive and selective. 12-14 Also, because E-AB sensors are electrochemistrybased, they are inherently fast, portable, and cost-effective. However, since E-AB relies on unique structures of aptamers, these sensors have to be designed case-by-case for different aptamertarget pairs. As a step further, Xiao et al. recently developed a potentially generalizable E-AB sensor for thrombin by using targetinduced strand displacement. 16 Here we report a target-responsive electrochemical aptamer switch (TREAS), which is a signal-on sensor featuring both generalizability and simplicity in design, toward reagentless detection of adenosine triphosphate (ATP) with high sensitivity and selectivity.We employed an in vitro selected 27-base anti-ATP aptamer, which possesses high affinity for ATP while not for its analogues, cytidine triphosphate (CTP), guanosine triphosphate (GTP), and uridine triphosphate (UTP). 17 The anti-ATP aptamer dually labeled with 3′-SH and 5′-ferrocene is self-assembled on gold electrodes in its duplex form (Scheme 1). We reason that ferrocene is distal to the electrode surface, thus cannot efficiently exchange electrons with the underlying electrodes due to large distance separation (∼10 nm) in this eT OFF state. In the presence of the target ATP, the tertiary aptamer structure is stabilized, which responsively denatures the duplex and liberates the complementary DNA, similar to the aptamer structural switch in solution. 18 As a consequence of this structural switch from the duplex to the tertiary aptamer structure (duplex-to-aptamer), the ferrocene moiety approaches the electrode surface and generates measurable electrochemical signals (eT ON). Of note, compared to the E-AB thrombin sensor reported by Xiao et al., 16 our TREAS is similarly generalizable while it has several advantages. First, the sensor architecture is simpler. The sensing DNA strand of E-AB contains three parts, aptamer region (for recognition), duplex region (structural support), and spacer region (linkage), thus is inherently longer than the corresponding TREAS sensing strand (only aptamer sequence). Second, TREAS has two we...

We herein report a novel nanoparticle-based electrochemical DNA detection approach. This DNA sensor is based on a "sandwich" detection strategy, which involves capture probe DNA immobilized on gold electrodes and reporter probe DNA labeled with gold nanoparticles that flank the target DNA sequence. Electrochemical signals are generated by chronocoulometric interrogation of [Ru(NH(3))(6)](3+) that quantitatively binds to surface-confined capture probe DNA via electrostatic interactions. We demonstrated that the incorporation of a gold nanoparticle in this sensor design significantly enhanced the sensitivity and the selectivity. Nanoscale control of the self-assembly process of DNA probes at gold electrodes further increased the sensor performance. As a result of these two combined effects, this DNA sensor could detect as low as femtomolar (zeptomoles) DNA targets and exhibited excellent selectivity against even a single-base mismatch. In addition, this novel DNA sensor showed fairly good reproducibility, stability, and reusability.

A novel bioassay strategy is designed to detect small-molecule targets such as cocaine, potassium, and adenosine, based on gold nanoparticles (AuNPs) and engineered DNA aptamers. In this design, an aptamer is engineered to be two pieces of random, coil-like single-stranded DNA, which reassembles into the intact aptamer tertiary structure in the presence of the specific target. AuNPs can effectively differentiate between these two states via their characteristic surface-plasmon resonance-based color change. Using this method, cocaine in the low-micromolar range is selectively detected within minutes. This strategy is also shown to be generic and applicable to the detection of several other small-molecule targets.