The Effects of Refractive Index Sensitivity and Chemical Interface Damping in Au–Ag Core–Shell

금, 은, 구리와 같은 귀금속 나노 입자는 국지 표면 플라즈몬 공명 (Localized Surface Plasmon resonance, LSPR) 현상에 의해 그들의 모양, 크기, 주변 매질의 굴절률(Refractive Index, RI) 등에 따라 독특한 광학적 특성을 가지며 그로 인해 다양한 분야에서 응용되고 있다. 그 중에서 금 및 은 나노 입자는 LSPR 바이오 센싱과 관련하여 가장 매력적인 물리적 특성을 보여준다. 금 나노 입자는 높은 화학적 안정성과 생체 적합성을 나타내므로 많은 분야에서 응용되고 있다. 은 나노 입자는 주어진 모양과 크기에 대해 금 나노 입자와 비교할 때 더 나은 RI 감도를 제공한다. 그럼에도 불구하고 은 나노 입자는 열등한 화학적 안정성 및 생체 적합성으로 인해 LSPR 기반 바이오 센싱 응용 분야에 사용될 때 수많은 한계를 보이는 것으로 되었지만, 최근에는 은 나노 입자와 금의 조합으로 많은 개선이 이루어졌다. 예를 들어, 금과 은을 혼합하여 합금 또는 Au-Ag 코어-쉘 나노 입자 (AuNRs @ Ag, core @ shell) 를 형성하면 Au-Ag 비율을 제어하여 지속적으로 조정할 수 있는 하이브리드 LSPR 밴드가 생성된다. 본 연구에서는 암시야(Dark-Field, DF) 현미경, 자외선-가시광선 분광기(UV-VIS spectroscope), 표면증강라만산란(Surface Enhanced Raman Scattering, SERS), 주사 전자 현미경(Scanning Electron Microscope, SEM)을 사용하여 AuNRs@Ag의 광특성을 분석하였다. 첫째, 쉘 두께가 다른 AuNRs@Ag에서 피리딘 유도체의 흡착 배향에 따른 화학적 계면 감쇠(Chemical Interface Damping, CID) 효과에 대해 알아보았다. 전자공여기를 갖는 피리딘 유도체는 단일 AuNRs@Ag에 흡착되어 질소와 은 표면의 상호 작용을 통해 강한 CID 효과를 유도하였다. AuNRs@Ag 두꺼운 은 쉘의 경우 모든 피리딘 유도체에 대해 얇은 은 쉘을 사용하는 AuNRs@Ag에 비해 상당히 향상된 CID 효과를 나타냈다. 피리딘 분자의 높은 기울기와 달리 electron donating group(EDG)을 포함하는 피리딘 유도체는 표면증강라만산란 및 동적 광산란 측정에 따라 Ag 표면에 평행한 배향을 채택하여 Ag 표면에서 피리딘과는 다른 표면 커버리지를 생성하고 LSPR 선폭확장이 감소됨을 확인하였다. 둘째, 금 코어의 크기는 같고 은 쉘의 두께가 다른 두가지의 AuNRs@Ag를 사용하여 주변 매질 굴절률에 따른 민감도(Refractive Index Sensitivity, RIS)에 은 쉘의 두께에 따른 영향을 알아보고 2차 미분에 의한 변곡점을 이용하여 RIS를 향상시켰다. 따라서 본 연구는 단일 AuNRs@Ag에서 피리딘 유도체의 공여 치환기와 Ag 쉘 두께가 CID에 미치는 영향에 대한 보다 깊은 이해를 제공하고 은 쉘 두께 증가로 인한 LSPR 센서의 개선을 보여준다. | Noble metal nanoparticles such as gold, silver, and copper have unique optical properties depending on their shape, size, and refractive index (RI) of the surrounding medium due to a localized surface plasmon resonance (LSPR) phenomenon. Among metallic NPs, gold (Au) and silver (Ag) nanoparticles(NPs) have demonstrated the most fascinating physical properties with regard to LSPR biosensing. These gold nanoparticles have been applied in many fields because they have the advantages of being biocompatible, having high chemical stability and easy surface modification. However, for a given shape and size, AgNPs provide better RI sensitivity when compared with AuNPs. Nevertheless, AgNPs have been reported to show numerous limitations when utilized for LSPR-based biosensing applications due to their inferior chemical stability and biocompatibility. Recently, many improvements have been achieved with the combination of AgNPs with Au. For instance, mixing Au and Ag to form either alloys or Au–Ag core–shell NPs (AuNRs@Ag , core @ shell) results in a hybrid LSPR band which can be tailored continuously by controlling the Au–Ag ratio. In this study, we used Dark-Field (DF) microscopy, UV-Vis spectroscopy (UV-Vis), Surface Enhanced Raman Scattering (SERS) and Scanning Electron Microscope (SEM) to analyze the optical properties of single AuNRs@Ag. First, the effects of chemical interface damping (CID) according to adsorption orientation of pyridine derivatives in AuNRs@Ag with different shell thicknesses were investigated. Pyridine derivatives having electron-donating groups (EDGs) were adsorbed on single AuNRs@Ag and induced a strong CID through the interaction of nitrogen with the Ag surface. AuNRs@Ag with thick shells showed a considerably enhanced CID effect compared with AuNRs@Ag with thin shells for all pyridine derivatives. In contrast to the high inclination of pyridine molecules, pyridine derivatives bearing EDGs adopted a parallel orientation to the Ag surface according to surface-enhanced Raman spectroscopy and dynamic light scattering measurements, which resulted in different surface coverage on the Ag surface and decreased LSPR linewidth broadening. Second, two AuNRs@Ag with the same size of the gold core and different thickness of the silver shell were used to investigate the effect of the thickness of the silver shell on the RI sensitivity according to the refractive index of the surrounding medium. RI sensitivity was improved by using the inflection point by the second derivative. Therefore, this study provides a deeper understanding of the effect of the donor substituent of the pyridine derivative and the Ag shell thickness on CID in a single AuNRs@Ag, and shows the improvement of the LSPR sensor due to the increase in silver shell thickness.Maste

Chemical Interface Damping of Silver-coated Gold Nanorods Using Supramolecular Host-Guest Chemistry

We present the chemical interface damping (CID) of single Ag-coated Au nanorods (AuNRs@Ag) using cucurbit[7]uril (CB[7])-based host-guest chemistry. The chemisorption of CB[7]-NH2 onto an AuNR@Ag surface resulted in a redshift of its localized surface plasmon resonance (LSPR) scattering peak, with a considerably increased linewidth. This LSPR broadening was ascribed to CID caused by the formation of a strong nitrogen-Ag interaction during the chemisorption of CB[7]-NH2 onto the AuNR@Ag surface. Furthermore, we observed additional broadening of the LSPR linewidth when the guest molecule (oxaliplatin) was encapsulated into CB[7]-NH2 chemisorbed onto the AuNR@Ag surface

Localized surface plasmon resonance inflection points for improved detection of chemisorption of 1-alkanethiols under total internal reflection scattering microscopy

Plasmonic gold nanoparticles are widely used in localized surface plasmon resonance (LSPR) sensing. When target molecules adsorb to the nanoparticles, they induce a shift in the LSPR scattering spectrum. In conventional LSPR sensing, this shift is monitored at the maximum of the LSPR scattering peak. Herein, we describe the sensitivity of detecting chemisorption of 1-alkanethiols with different chain lengths (1-butanethiol and 1-haxanethiol) on single gold nanorods (AuNRs) of fixed diameter (25 nm) and three different aspect ratios under a total internal reflection scattering microscope. For single AuNRs of all sizes, the inflection point (IF) at the long-wavelength side (or low-energy side) of the LSPR scattering peak showed higher detection sensitivity than the traditionally used peak maximum. The improved sensitivity can be ascribed to the shape change of the LSPR peak when the local refractive index is increased by chemisorption. Our results demonstrate the usefulness of tracking the curvature shapes by monitoring the homogeneous LSPR IF at the red side of the scattering spectrum of single AuNRs

In situ reversible tuning of chemical interface damping in single gold nanorod-based recyclable platforms through manipulation of supramolecular host-guest interactions

Recently, chemical interface damping (CID) has been proposed as a new plasmon damping pathway based on interfacial hot-electron transfer from metal to adsorbate molecules. It has been considered essential, owing to its potential implications in efficient photochemical processes and sensing experiments. However, thus far, studies focusing on controlling CID in single gold nanoparticles have been very limited, and in situ reversible tuning has remained a considerable challenge. In these scanning electron microscopy-correlated dark-field spectroscopic measurements and density functional theory calculations, cucurbit[7]uril (CB[7])-based host-guest supramolecular interactions were employed to examine and control the CID process using monoamine-functionalized CB[7] (CB[7]-NH2) attached to single gold nanorods (AuNRs). In situ tuning of CID through the CB[7]-oxaliplatin complexation, which can result in the variation of the chemical nature and electronic properties of adsorbates, was presented. In addition, in situ tuning of CID was demonstrated through the competitive release of the oxaliplatin guest from the oxaliplatin@CB[7] complex, which was then replaced by a competitor guest of spermine in sufficient amounts. Furthermore, nuclear magnetic resonance experiments confirmed that the release of the guest is the consequence of adding salt (NaCl). Thus, in situ reversible tuning of CID in single AuNRs was achieved through successive steps of encapsulation and release of the guest on the same AuNR in a flow cell. Finally, single CB[7]-NH2@AuNRs were presented as a recyclable platform for CID investigations after the complete release of guest molecules from their host-guest inclusion complexes. Therefore, this study has paved a new route to achieve in situ reversible tuning of CID in the same AuNR and to investigate the CID process using CB-based host-guest chemistry with various guest molecules in single AuNRs for efficient hot-electron photochemistry and biosensing applications