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.

A rapid immunochromatographic lateral flow test strip of competitive format has been developed for the specific determination of olaquindox (OLA) residues in pig urine and muscle tissues. The sensitivity of the test strip was found to be 1.58 ± 0.27 μg/kg and 1.70 ± 0.26 μg/kg of OLA in pig urine and muscle tissues, and the lower detection limit was 0.27 ± 0.08 μg/kg and 0.31 ± 0.07 μg/kg respectively. For negative pig urine and muscle samples spiked with 4, 12, and 36 μg/kg, the recovery range was 83.0-94.0% and 78.8-87.4% and the coefficient of variation scope [CV (%)] was 3.17-7.41% and 4.66-7.64% respectively. Parallel analysis of OLA samples from pig urine and muscle tissue showed comparable results from the test strip and HPLC. Each test requires 5-8 min, and the test strip can provide a useful screening method for quantitative, semiquantitative, or qualitative detection of OLA residues.

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 nanorod (GNR) is an attractive optical transducer for label-free biosensing owing to the localized surface plasmon resonance (LSPR) which is highly sensitive to the dielectric constant of the surrounding medium modulated by biological bindings. By adjusting the nanorod aspect ratio (length to width ratio), desired absorption wavelength can be continuously tuned from 600 to 1,100 nm. Here we demonstrated a linear relationship between the aspect ratio and the LSPR peak wavelength. Taking advantage of this tunability feature, we developed a multiplexed GNR sensor by combining nanorods with distinct LSPR wavelengths. Specifically, GNRs of AR 2.1 and 4.2 exhibiting longitudinal plasmonic band of 640 and 830 nm respectively were functionalized with specific antibody. Concentrations of multiple analytes were measured by correlating to the spectral shift at the distinct plasmon band maxima upon specific binding. The practical use of this mixed bioprobes for simultaneous quantification of cardiac biomarkers (myoglobin and cardiac troponin I) in the clinically significant sensing range was described. The LSPR red shift magnitude is linearly proportional to the increase in the target analyte concentration (R2 = 0.98). The calibration curve can clearly differentiate varying biomarker amounts with a high specificity. For multiplexed biosensing, the plasmon shift at the dedicated peak wavelength can be specifically correlated with spiked biomarker for simultaneous detection in the sample mixture. This technology can be further transformed onto miniaturized biochips based on the nanosized optical transducer to allow point-of-care blood testing for risk stratifications of cardiac patients in clinical settings.

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.