We demonstrate that Fe3O4 magnetic nanoparticle (MNP) can greatly enhance the localized surface plasmon resonance (LSPR) of metal nanoparticle. The high refractive index and molecular weight of the Fe3O4 MNPs make them a powerful enhancer for plasmonic response to biological binding events, thereby enabling a significant improvement in the sensitivity, reliability, dynamic range, and calibration linearity for LSPR assay of small molecules in trace amount. Rather than using fluorescence spectroscopy or magnetic resonance imaging, this study marks the first use of the label-free LSPR nanosensor for a disease biomarker in physiological solutions, providing a low cost, clinical-oriented detection. This facile and ultrasensitive nanosensor with extremely light, robust, and low-cost instrument is attractive for miniaturization on a lab-on-a-chip system to deliver point-of-care medical diagnostics. To further evaluate the practical application of Fe3O4 MNPs in the enhancement of LSPR assay, cardiac troponin I (cTnI) for myocardial infarction diagnosis was used as a model protein to be detected by a gold nanorod (GNR) bioprobe. MNP-captured cTnI molecules resulted in spectral responses up to 6 fold higher than direct cTnI adsorption on the GNR sensor. The detection limit (LOD) was lowered to ca. 30 pM for plasma samples which is 3 orders lower than comparable study. To the best of our knowledge, this marks the lowest LOD for a real plasma protein detection based on label-free LSPR shift without complicated instrumentation. The observed LSPR sensing enhancement by Fe3O4 MNPs is independent of nonspecific binding.

Surface modification of gold nanorods (GNRs) is often problematic due to tightly packed cetyltrimethylammoniumbromide (CTAB) bilayer. Herein, we performed a double phase transfer ligand exchange to achieve displacement of CTAB on nanorods. During the removal, 11-mercaptoundecanoic acid (MUDA) crosslinker is simultaneously assembled on nanorod surfaces to prevent aggregation. The resulting MUDA-GNRs retain the shape and position of plasmon peaks similar to CTAB-capped GNRs. The introduction of carboxyl groups allows covalent conjugation of biological receptors in a facile fashion to construct a robust, label-free biosensor based on localized surface plasmon resonance (LSPR) transduction of biomolecular interaction. More importantly, smaller MUDA layer on the GNRs reduces the distance of target binding to the plasmonic nanostructure interface, leading to a significant enhancement in LSPR assay sensitivity and specificity. Compared to modification using conventional electropolymer adsorption, MUDA-coated gold nanosensor exhibits five times lower detection limit for cardiac troponin I assay with a high selectivity.

Gold nanorods with localized surface plasmon resonance (LSPR) can be chemically synthesized. We systematically investigated the effects of reaction parameters and centrifugation on the fine tuning of the rod dimension in scale-up production (80-100 mL). Nanorods of absorption bands from 600-1050 nm were fabricated with precise control of the aspect ratio (AR) from 1.5 to 8.9. Although all chemicals are important in directing the nanostructure, silver ion concentration and seed/Au 3+ ratio were the most effective variations to adjust the absorption wavelength. With a single surfactant under the influence of silver nitrate, short nanorods up to AR of 5 were synthesized with corresponding maximum absorption wavelength at 902 nm. To achieve higher aspect ratio with absorption band beyond 1,000 nm, two-surfactant growth solution was sought to further elongate the rod length. Centrifugation speed and times were found to exert significant influences on the final rod dimension, which is important during the purification process. In a relatively large quantity nanorod synthesis, even distribution and sufficient mixing of chemical ingredients play an essential role in determining the yield, uniformity, and stability of the final nanorod formation.

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In this report, we demonstrate a nanoscale biosensor required for in field and fast detection. This sensor utilizes the Surface Plasmon Resonance (SPR) property of the gold nanorods of aspect ratio 5 to amplify the bio detection whose response is captured by three different photodiodes fabricated by the TSMC 0.18µm. Pinned photodiode yielded 176% and 80% higher response current than N+ diffusion/ P substrate (N+/psub) and N well/P substrate (NW/psub) diodes respectively. However NW/psub diode exhibited 1.5 and 6.7 times more sensitivity to 20nm shift in gold nanorod spectrum when compared to pinned and N+/psub diodes respectively.