In this work, bifunctional core@shell Au@Pt/Au NPs are presented as novel tags for electrochemical immunosensing.Au@Pt/Au NPs were synthesized following a chemical route based on successive metal depositions and galvanic replacement reactions from the starting AuNPs. Au protuberances growth on the surface of Au@Pt NPs allowed their easy bioconjugation with antibodies, while the high catalytic Pt surface area was approached for their sensitive detection through the electrocatalysed water oxidation reaction (WOR) at neutral pH. Moreover, the synergy between Au and Pt metals on the NP surface also lead to an increased catalytic activity, improving the sensitivity of the NP detection. Cyclic voltammetry and chronoamperometry were used for the evaluation of the Au@Pt/Au NPs electrocatalytic activity towards WOR. The chronoamperometric current recorded at a fixed potential of +1.35 V was selected as the analytical signal, allowing the quantification of Au@Pt/Au NPs at 10 13 NPs/mL levels. The optimized electrocatalytic method was applied to the quantification of conformationally altered p53 peptide Alzheimer's disease (AD) biomarker in a competitive immunoassay using magnetic bead (MB) platforms at levels as low as 66 nM. The performance of the system in a real scenario was demonstrated analysing plasma samples from a cognitively healthy subject. This novel Au@Pt/Au NPs-based electrocatalytic immunoassay has the advantage, over common methods for NP tags electrochemical detection, of the signal generation in the same neutral medium where the immunoassay takes place (0.1 M PBS pH 7.2), avoiding the use of additional and more hazardous reagents and paving the way to future integrated biosensing systems. 38 trolled synthesis, higher stability against harsh conditions, 39 higher resistance to high concentrations of substrate and a lower 40 cost [3][4][5][6] . The use of electrocatalytic NPs as labels has been ex-41 tensively studied and applied in immunosensing [7][8][9][10][11][12] , offering 42 outstanding alternatives to traditional assays. 43Among the wide variety of NPs, metallic NP labels have at-44 tracted considerable interest due to their unique red-ox and op-45 tical properties 13,14 as well as their electrocatalytic activity, also 46 benefiting of the inherent advantages of the electrochemical de-47 tection in terms of sensitivity, selectivity, simplicity and low 48 cost 15 . In most cases highly acidic media are needed for such 49 NPs detection, either to facilitate dissolution 16 or as source of 50 hydrogen ions for further detection based on hydrogen evolu-51 tion reaction (HER) [17][18][19][20][21][22][23] . However, the use of acid solutions is 52 not desirable for both safety reasons and the time needed for the 53 analysis, also involving additional steps after the immunoassay. 54Consequently, there is a need of NP tags that may be detected 55 in the same medium where immunoreactions take place. In this 56 context, the water oxidation reaction (WOR) occurring at neu-57 tral pH and easily catalysed by some ...

This review shows recent trends in the use of nanoparticles as labels for electrochemical immunosensing applications. Some general considerations on the principles of both the direct detection based on redox properties and indirect detection through electrocatalytic properties, before focusing on the applications for mainly proteins detection, are given. Emerging use as blocking tags in nanochannels-based immunosensing systems is also covered in this review. Finally, aspects related to the analytical performance of the developed devices together with prospects for future improvements and applications are discussed.

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The sensitive monitoring of mercury (II) selenide nanoparticles (HgSe NPs) is of great potential relevance in environmental studies, since such NPs are believed to be the ultimate metabolic product of the lifesaving mechanism pathway of Hg detoxification in biological systems. In this context, we take advantage of using gold-nanostructured screen-printed carbon electrodes (SPCE-Au) for the rapid, simple and sensitive electrochemical quantification of engineered water-stable HgSe NPs, as an advantageous alternative to conventional elemental analysis techniques. HgSe NPs are first treated in an optimized oxidative/acidic medium for Hg 2+ release, followed by sensitive electrochemical detection by anodic stripping voltammetry (ASV). To the best of our knowledge, this is the first time that water-stable HgSe NPs are quantified using electrochemical techniques. The low limit of detection achieved (3.86 × 10 7 HgSe NPs/mL) together with the excellent repeatability (RSD: 3%), reproducibility (RSD: 5%) and trueness (relative error: 10%), the good performance in real sea water samples (recoveries of the analytical signal higher than 90%) and the simplicity/low cost of analysis make our method as ideal candidate for HgSe NPs monitoring in future environmental studies.

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