As(III) specifically interacts with an arsenic-binding aptamer to form an As(III)–aptamer complex, so that the following cationic polymer can aggregate gold nanoparticles (AuNPs) and cause a remarkable change in color, which enables the colorimetric detection of As(III) with high selectivity and a detection limit of 5.3 ppb.

The demand for selection of aptamers against various small chemical molecules has substantially increased in recent years. To incubate and separate target-specific aptamers, the conventional SELEX procedures generally need to immobilize target molecules on a matrix, which may be impotent to screen aptamers toward small molecules without enough sites for immobilization. Herein we chose Cd(II) as a model of a small molecule with less sites, and proposed a novel SELEX strategy of immobilizing ssDNA libraries rather than target molecules on a matrix, for selection of aptamers with high affinity to Cd(II). After eleven rounds of positive and negative selection, twelve T and G-rich of nonrepeating ssDNA sequences were identified, of which the Cd-4 aptamer displayed the highest binding affinity to Cd(II). The secondary structures of these sequences revealed that a stem-loop structure folded by the domain of their 30-random sequence is critical for aptamers to bind targets. Then the interaction between the selected Cd-4 aptamer and Cd(II) was confirmed by CD analysis, and the binding specificity toward other competitive metal ions was also investigated. The dissociation constant (K_d) of Cd-4 aptamer was determined as 34.5 nM for Cd(II). Moreover, the Cd-4 aptamer was considered a recognition element for the colorimetric detection of Cd(II) based on the aggregation of AuNPs by cationic polymer. Through spectroscopic quantitative analysis, Cd(II) in aqueous solution can be detected as low as 4.6 nM. The selected Cd-4 aptamer will offer a new substitute for the detection of Cd(II) or other applications like recovery of cadmium from polluted samples.

This paper reports the colorimetric and resonance scattering (RS)-based biosensor for the ultrasensitive detection of As(III) in aqueous solution via aggregating gold nanoparticles (AuNPs) by the special interactions between arsenic-binding aptamer, target and cationic surfactant. Aptamers and the cationic surfactant could assemble to form a supramolecule, which prevented AuNPs from aggregating due to the exhaustion of cationic surfactant. The introduction of As(III) specifically interacted with the arsenic-binding aptamer to form the aptamer–As(III) complex, so that the following cationic surfactant could aggregate AuNPs and cause the remarkable change in color and RS intensity. The results of circular dichroism (CD) and scanning probe microscope (SPM) testified to the formation of the supramolecule and aptamer–As(III) complex, and the observation of transmission electron microscope (TEM) further confirmed that the aggregation of AuNPs could be controlled by the interactions among the aptamer, As(III) and cationic surfactant. The variations of absorbance and RS intensity were exponentially related to the concentration of As(III) in the range from 1 to 1500 ppb, with the detection limit of 40 ppb for the naked eye, 0.6 ppb for colorimetric assay and 0.77 ppb for RS assay. Additionally, the speed of the present biosensor was rapid, and it also exhibited high selectivity over other metal ions with an excellent recovery for detection in real water samples, suggesting that the proposed biosensor will play an important role in environmental detection.

Aptamer-assembled nanomaterials have captured much attention from the field of analytical chemistry in recent years. Although they have been regarded as a promising tool for heavy metal monitoring, report involving aptamer-based biosensors for arsenic detection are rare. Herein we developed a highly sensitive and selective aptamer biosensor for As(III) detection based on a Resonance Rayleigh Scattering (RRS) spectral assay. Prior to As(III) detection, we firstly assembled a variety of nanoparticles with different sizes via controlling the concentration of arsenic-binding aptamers in crystal violet (CV) solutions. The results of photon correlation spectroscopy (PCS) and scanning probe microscope (SPM) testified that the introduction of As(III) had indeed changed the size of nanoparticles, which caused a great variation in the RRS intensity at 310 nm. In the presence of 100 ppb As(III), a maximum decline in the ratio of RRS intensity was achieved for large nanoparticles assembled from 200 nM of aptamers and CV molecules, where the average size of nanoparticles had decreased from 273 nm to 168 nm. In the case of small nanoparticles, the maximum increase ratio of the RRS intensity was obtained when the concentration of aptamer was over 600 nM. Combined with an RRS spectral assay, an effective biosensor has been developed for As(III) detection, using the above large and small nanoparticles as the target recognition element. The present biosensor has a detection limit as low as 0.2 ppb, a dynamic range from 0.1 ppb to 200 ppb, and high selectivity over other metal ions. Such an efficient biosensor will play an important role in environmental detection.