In this paper, an asymmetric split-ring resonator, metamaterial element, is presented as a biosensing transducer for detection of highly sensitive and label-free stress biomarkers. In particular, the two biomarkers, cortisol and α-amylase, are used for evaluating the sensitivity of the proposed biosensor. In case of cortisol detection, the competitive reaction between cortisol-bovine serum albumin and free cortisol is employed, while alpha-amylase is directly detected by its antigen-antibody reaction. From the experimental results, we find that the limit of detection and sensitivity of the proposed sensing device are about 1 ng/ml and 1.155 MHz/ng ml−1, respectively.
The phase evolution of as-prepared NaYF4:Yb,Er upconversion nanoparticles (UCNPs) with a metastable cubic structure is studied based on in situ heating experiments via transmission electron microscopy (TEM). The atomistic behavior on the single NaYF4:Yb,Er UCNP is observed during the phase transition. The formation and evolution of voids on the NaYF4:Yb,Er UCNP appear at a temperature below 420 °C. Small circular voids are transformed at the initial stage to a large, hexagonal-pillar shaped single void. Two different routes to reach the stable α-phase from the metastable cubic structure are identified on a single NaYF4:Yb,Er UCNP. The first is via a stable β-phase and the second is a direct change via a liquid-like phase. The specific orientation relationships, [110]cubic//[110]hexagonal and {002}cubic//{200}hexagonal, between the cubic and hexagonal structures are confirmed. Additionally, a few extra-half planes terminated in the cubic structures are also observed at the cubic/hexagonal interface.
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