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Reproducible electrochemically encoded quantum dot (QD) barcodes were prepared using the reverse-micelle synthetic approach. The encoding elements, [Zn.sup.2+], [Cd.sup.2+], and [Pb.sup.2+], were confined within a single QD, which eliminates the cumbersome encapsulation process used by other common nanoparticle-based barcode preparation schemes. The distinct voltammetric stripping patterns of [Zn.sup.2+], [Cd.sup.2+] and [Pb.sup.2+] at distinguishable potentials with controllable current intensities offer excellent encoding capability for the prepared electrochemical (EC) QDs. Additionally, the simultaneous modification of the QD barcode surface with organic ligands during the preparation process make them potentially useful in biomedical research. For proof of concept of their application in bioassays, the EC QD barcodes were further employed as tags for an immunoassay of a cancer marker, carcinoembryonic antigen (CEA). The voltammetrlc stripping response of the dissolved bardcode tags was proportional to log[CEA] in the range from 0.01 to 80 ng [mL.sup.-1], with a detection limit of 3.3 pg [mL.sup.-1]. The synthesized EC QD barcodes hold considerable potential in biodetection, encrypted information, and product tracking. 10.1021/ac902335e

A new signal amplification strategy based on thionine (TH)-doped magnetic gold nanospheres as labels and horseradish peroxidase (HRP) as enhancer holds promise to improve the sensitivity and detection limit of the immunoassay for carcinoembryonic antigen (CEA), as a model protein. This immunoassay system was fabricated on a carbon fiber microelectrode (CFME) covered with a well-ordered anti-CEA/protein A/nanogold architecture. The reverse micelle method was initially used for the preparation of TH-doped magnetic gold nanospheres (nanospheres), and the synthesized nanospheres were then labeled on HRP-bound anti-CEA as a secondary antibody (bionanospheres). Sandwich-type protocol was successfully introduced to develop a new high-efficiency electrochemical immunoassay with the labeled bionanospheres toward the reduction of [H.sub.2][O.sub.2]. Under optimized conditions, the linear range of the proposed immunoassay without HRP as enhancer was 1.2-125 ng/mL CEA, whereas the assay sensitivity by using HRP as enhancer could be further increased to 0.01 ng/mL with the linear range from 0.01 to 160 ng/mL CEA. The developed immunoassay method showed good precision, high sensitivity, acceptable stability and reproducibility, and could be used for the detection of real samples with consistent results in comparison with those obtained by the enzyme-linked immunosorbent assay (ELISA) method.