Unique electronic and ligand recognition properties of the DNA double helix provide basis for DNA applications in biomolecular electronic and biosensor devices. However, the relation between the structure of DNA at electrified interfaces and its electronic properties is still not well understood. Here, potential-driven changes in the submolecular structure of DNA double helices composed of either adenine-thymine (dAdT) or cytosine-guanine (dGdC) base pairs tethered to the gold electrodes are for the first time analyzed by in situ polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) performed under the electrochemical control. It is shown that the conformation of the DNA duplexes tethered to gold electrodes via the C alkanethiol linker strongly depends on the nucleic acid sequence composition. The tilt of purine and pyrimidine rings of the complementary base pairs (dAdT and dGdC) depends on the potential applied to the electrode. By contrast, neither the conformation nor orientation of the ionic in character phosphate-sugar backbone is affected by the electrode potentials. At potentials more positive than the potential of zero charge (pzc), a gradual tilting of the double helix is observed. In this tilted orientation, the planes of the complementary purine and pyrimidine rings lie ideally parallel to each other. These potentials do not affect the integral stability of the DNA double helix at the charged interface. At potentials more negative than the pzc, DNA helices adopt a vertical to the gold surface orientation. Tilt of the purine and pyrimidine rings depends on the composition of the double helix. In monolayers composed of (dAdT) molecules the rings of the complementary base pairs lie parallel to each other. By contrast, the tilt of purine and pyrimidine rings in (dGdC) helices depends on the potential applied to the electrode. Such potential-induced mobility of the complementary base pairs can destabilize the helix structure at a submolecular level. These pioneer results on the potential-driven changes in the submolecular structure of double stranded DNA adsorbed on conductive supports contribute to further understanding of the potential-driven sequence-specific electronic properties of surface-tethered oligonucleotides.

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Electrical properties of DNA critically depend on the way DNA molecules are integrated within the electronics, particularly on DNA-electrode immobilization strategies. Here, we show that the rate of electron transport in DNA duplexes spacer-free tethered to gold via the adenosine terminal region (a dA tag) is enhanced compared to the hitherto reported DNA-metal electrode tethering chemistries. The rate of DNA-mediated electron transfer (ET) between the electrode and methylene blue intercalated into the dA-tagged DNA duplex approached 361 s at a ca. half-monolayer DNA surface coverage Γ (with a linear regression limit of 670 s at Γ → 0), being 2.7-fold enhanced compared to phosphorothioated dA* tethering (6-fold for the C-alkanethiol linker representing an additional ET barrier). X-ray photoelectron spectroscopy evidenced dA binding to the Au surface via the purine N, whereas dA* predominantly coordinated to the surface via sulfur atoms of phosphothioates. The latter apparently induces the DNA strand twist in the point of surface attachment affecting the local DNA conformation and, as a result, decreasing the ET rates through the duplex. Thus, a spacer-free DNA coupling to electrodes via dA tags thus allows a perspective design of DNA electronic circuits and sensors with advanced electronic properties and no implication from more expensive, synthetic linkers.

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.

Specific and sensitive electroanalysis of blood‐circulating protein cancer biomarkers is often complicated by interference from serum proteins nonspecifically adsorbing at the biosensing interface and masking specific reactions of interest. Here, we have developed an electrocatalytically amplified assay for specific and sensitive analysis of human epidermal growth factor receptor‐2 (HER‐2/neu, a protein cancer biomarker over‐expressed in breast cancers) that allows us to avoid both the interference from bovine serum albumin (BSA) and electrocatalytic amplification of the signal stemming from the specific aptamer−HER‐2/neu binding. A HER‐2/neu‐specific thiolated aptamer sequence was co‐adsorbed on gold together with a C11 alkanethiol bearing two ethylene glycol (EG)2 head groups that prevented non‐specific adsorption of BSA. On such layers, electrochemical reduction of a ferricyanide redox indicator is inhibited and is shown to be electrocatalyzed by methylene blue electrostatically interacting with negatively charged HER‐2/neu. The electrocatalytic signal increased upon HER‐2/neu binding to the aptamer, which allowed 10−12–10−8 M HER‐2/neu detection in 1 % serum, being practically applicable for clinical testing. The developed strategy can be considered as general and applicable for the electroanalysis of other blood‐circulating proteins once the electrostatic compatibility between the protein and redox probe is established.

The drive toward sustainable phosphorus (P) recovery from agricultural and municipal wastewater streams has intensified. However, combining P recovery with energy conservation is perhaps one of the greatest challenges of this century. In this study, we report for the first time the simultaneous electroless production of struvite and dihydrogen from aqueous ammonium dihydrogen phosphate (NH4H2PO4) solutions in contact with either a pure magnesium (Mg) or a Mg alloy as the anode and 316 stainless steel (SS) as the cathode placed in a bench-scale electrochemical reactor. During the electroless process (i.e., in the absence of external electrical power), the open circuit potential (OCP), the formation of struvite on the anode, and the generation of dihydrogen at the cathode were monitored. We found that struvite is formed, and that struvite crystal structure/morphology and precipitate film thickness are affected by the concentration of the H n PO4 n–3/NH4 + in solution and the composition of the anode. The pure Mg anode produced a porous 0.6–4.1 μm thick film, while the AZ31 Mg alloy produced a more compact 1.7–9.9 μm thick struvite film. Kinetic analyses revealed that Mg dissolution to Mg2+ followed mostly a zero-order kinetic rate law for both Mg anode materials, and the rate constants (k) depended upon the struvite layer morphology. Fourier-transform infrared spectrometry, X-ray diffraction, and scanning electron microscopy indicated that the synthesized struvite was of high quality. The dihydrogen and Mg2+ in solution were detected by a gas chromatography–thermal conductivity detector and ion chromatography, respectively. Furthermore, we fully demonstrate that the reactor was able to remove ∼73% of the H n PO4 n–3 present in a natural poultry wastewater as mainly struvite. This study highlights the feasibility of simultaneously producing struvite and dihydrogen from wastewater effluents with no energy input in a green and sustainable approach.

Charges of redox species can critically affect both the interfacial state of DNA and electrochemistry of DNA-conjugated redox labels and, as a result, the electroanalytical performance of those systems. Here, we show that the kinetics of electron transfer (ET) between the gold electrode and methylene blue (MB) label conjugated to a double-stranded (ds) DNA tethered to gold strongly depend on the charge of the MB molecule, and that affects the performance of genosensors exploiting MB-labeled hairpin DNA beacons. Positively charged MB binds to dsDNA via electrostatic and intercalative/groove binding, and this binding allows the DNA-mediated electrochemistry of MB intercalated into the duplex and, as a result, a complex mode of the electrochemical signal change upon hairpin hybridization to the target DNA, dominated by the "on-off" signal change mode at nanomolar levels of the analyzed DNA. When MB bears an additional carboxylic group, the negative charge provided by this group prevents intimate interactions between MB and DNA, and then the ET in duplexes is limited by the diffusion of the MB-conjugated dsDNA (the phenomenon first shown in Farjami , E. ; Clima , L. ; Gothelf , K. ; Ferapontova , E. E. Anal. Chem. 2011 , 83 , 1594 ) providing the robust "off-on" nanomolar DNA sensing. Those results can be extended to other intercalating redox probes and are of strategic importance for design and development of electrochemical hybridization sensors exploiting DNA nanoswitchable architectures.

Bimetallic iron−nickel oxide/hydroxide (FeNiO(H) x ) nanocatalysts have emerged as nonprecious metal candidates for alkaline oxygen evolution reaction (OER) electrocatalysis. However, there are still significant open questions regarding the role of electrocatalyst synthesis route, and the resulting electrocatalyst morphology and nanoscale structure, in determining the operando atomicscale structure when subjected to the faradic OER voltage environment. Herein, we report on two nanoparticle FeNiO(H) x electrocatalysts and their different chemical structures using operando X-ray absorption spectroscopy (XAS) studies at relevant OER conditions. The two bimetallic nanoparticle electrocatalysts were synthesized using aqueous (NP-aq) vs oil-based (NP-oil) synthesis routes but resulted in compositionally similar surface chemistry as-synthesized. Operando XAS results suggest that Ni oxidizes from the initial +2 oxidation state to +3/+4 state reminiscent of the transformation of α-Ni(OH) 2 to γ-NiOOH; the oxidation state change is voltage-dependent and occurs in both NP-aq and NP-oil nanoparticles. There does not appear to be an oxidation state change for Fe, but the Fe coordination environment does change with voltage. The NP-aq nanoparticles resulted in Fe coordination transitions between Fe 3+ T d , observed in as-synthesized and 0.8−0.9 V vs Ag/AgCl conditions, and Fe 3+ O h , observed at 0 V vs Ag/AgCl, while the NP-oil nanoparticles resulted in a largely stable Fe 3+ O h coordination with more subtle changes in the coordination environment. The voltage dependence of this Fe coordination transition is nanoparticle-dependent, with NP-aq nanoparticles transitioning dramatically at 0.7 V vs Ag/AgCl but NP-oil nanoparticles transitioning slowly starting at 0.1 V vs Ag/ AgCl. Additionally, a shortening of both the Fe−O and Ni−O bond distances occurs for both nanoparticle materials, but the magnitude of change is different for NP-aq vs NP-oil, suggesting that the nanoparticle structures result in unique changes under applied potential. Extended X-ray absorption fine structure (EXAFS) analysis showed distinct chemical environments for the Fe species of NP-aq vs NP-oil, metallic Fe and Ni character in NP-aq, and Ni largely in a hydroxide phase for both nanoparticles. NP-aq results in improved activity and stability during OER, as compared to NP-oil, suggesting that the Fe 3+ O h → T d transition, metallic core, and a predominant Fe-incorporated Ni(OH) 2 phase in the shell are important for OER performance. This study highlights that both the electrochemical environment and the as-synthesized morphology of nanoparticle electrocatalysts are important in determining the operational chemical structures and structure−performance relationships.