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Recently nucleic acids gained considerable attention as selective receptors of metal ions. This is because of the possibility of adjusting their sequences in new aptamers selection, as well as the convenience of elaborating new detection mechanisms. Such a flexibility allows for easy utilization of newly emerging nanomaterials for the development of detection devices. This, in turn, can significantly increase, e.g., analytical signal intensity, both optical and electrochemical, and the same can allow for obtaining exceptionally low detection limits and fast biosensor responses. All these properties, together with low power consumption, make nucleic acids biosensors perfect candidates as detection elements of fully automatic portable microfluidic devices. This review provides current progress in nucleic acids application in monitoring environmentally and clinically important metal ions in the electrochemical or optical manner. In addition, several examples of such biosensor applications in portable microfluidic devices are shown.

The growing number of various diseases and the increase of environmental contamination are the causes for the development of novel methods for their detection. The possibility of the application of affinity-based biosensors for such purposes seems particularly promising as they provide high selectivity and low detection limits. Recently, the usage of hybrid antibody-aptamer sandwich constructs was shown to be more advantageous in terms of working parameters in comparison to aptamer-based and immune-based biosensors. This review is focused on the usage of hybrid antibody-aptamer receptor layers for the determination of clinically and environmentally important target molecules. In this work, antibodies and aptamer molecules are characterized and the methods of their immobilization as well as analytical signal generation are shown. This is followed by the critical presentation of examples of hybrid sandwich biosensors that have been elaborated in the past 12 years.

C-reactive protein (CRP) is a crucial biomarker of cardiovascular diseases and for its detection both optical and electrochemical techniques were applied. This study concerns the application of DNA aptamer as recognition layer for CRP detection. For that purpose aptamer immobilization method on gold surface was selected and the content of receptor layer was optimized to ensure an efficient binding to target protein. The quality of the monolayer was verified by the application of chronocoulometry and atomic force microscopy. Using thiolated aptamers provided the formation of layers of highest density and stability. The square-wave voltammetry experiments performed in the presence of methylene blue redox indicator revealed a linear response of aptasensor towards CRP in the range from 1 to 100 pM. Moreover, a DNA aptamer – based sensor showed good selectivity towards C-reactive protein in comparison to interfering proteins including BSA and IgE. Finally, the analysis of CRP in serum sample was conducted using the developed aptasensor.

This paper focuses on the current state of art as well as on future trends in electrochemical aptasensors application in medical diagnostics. The origin of aptamers is presented along with the description of the process known as SELEX. This is followed by the description of the broad spectrum of aptamer-based sensors for the electrochemical detection of various diagnostically relevant analytes, including metal cations, abused drugs, neurotransmitters, cancer, cardiac and coagulation biomarkers, circulating tumor cells, and viruses. We described also possible future perspectives of aptasensors development. This concerns (i) the approaches to lowering the detection limit and improvement of the electrochemical aptasensors selectivity by application of the hybrid aptamer–antibody receptor layers and/or nanomaterials; and (ii) electrochemical aptasensors integration with more advanced microfluidic devices as user-friendly medical instruments for medical diagnostic of the future.

The possibility of utilization of gold electrodes modified with short guanine-rich ssDNA probes for determination of Pb(2+) was examined. Interaction between guanine residues and lead ion followed by formation of G-quadruplex structures was confirmed by electrochemical impedance spectroscopy investigations. An external cationic redox label, methylene blue, was employed in voltammetric measurements for analytical signal generation. It was shown that due to the G-quadruplex formation, the oligonucleotides in the recognition layer fold, which enhances the electron transfer between methylene blue and the electrode surface. The MB current signal rises proportionally to the lead ion concentration in the range from 0.05 to 1μmol/L. The developed biosensor demonstrated high selectivity towards Pb(2+) ion, with only minor response towards interfering metal cations. The calculated limit of detection was of 34.7nmol/L. The utilization of the biosensor for Pb(2+) determination in real samples of water was also tested.

The present study aims at the further development of the uranyl oligonucleotide-based voltammetric biosensor, which takes advantage of strong interaction between UO2(2+) and phosphate DNA backbone. Herein we report the optimization of working parameters of previously elaborated electrochemical DNA biosensor. It is shown that the sensor sensitivity is highly dependent on the oligonucleotide probe length and the incubation time of sensor in a sample solution. Consequently, the highest sensitivity was obtained for 10-nucleotide sequence and 60 min incubation time. The lower detection limit towards uranyl cation for developed biosensor was 30 nM. The influence of mixed monolayers and the possibility of developing a non-calibration device were also investigated. The selectivity of the proposed biosensor was significantly improved via elimination of adenine nucleobases from the DNA probe. Moreover, the regeneration procedure was elaborated and tested to prolong the use of the same biosensor for 4 subsequent determinations of UO2(2+).

Protein biomarkers of cancer allow a dramatic improvement in cancer diagnostics as compared to the traditional histological characterisation of tumours by enabling a non-invasive analysis of cancer development and treatment. Here, an electrochemical label-free assay for urokinase plasminogen activator (uPA), a universal biomarker of several cancers, has been developed based on the recently selected uPA-specific fluorinated RNA aptamer, tethered to a gold electrode via a phosphorothioated dA tag, and soluble redox indicators. The binding properties of the uPA-aptamer couple and interference from the non-specific adsorption of bovine serum albumin (BSA) were modulated by the electrode surface charge. A nM uPA electroanalysis at positively charged surfaces, complicated by the competitive adsorption of BSA, was tuned to the pM uPA analysis at negative surface charges of the electrode, being improved in the presence of negatively charged BSA. The aptamer affinity for uPA displayed via the binding/dissociation constant relationship correspondingly increased, ca. three orders of magnitude, from 0.441 to 367. Under optimal conditions, the aptasensor allowed 10(-12)-10(-9) M uPA analysis, also in serum, being practically useful for clinical applications. The proposed strategy for optimization of the electrochemical protein sensing is of particular importance for the assessment and optimization of in vivo protein ligand binding by surface-tethered aptamers.