Unraveling a role of molecular linker in nanoparticles self-organization by SERS spectroscopy: Comparative study of three aromatic diamines

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Abstract

The preparation of controllable small noble metal nanoparticle (NP) aggregates is required for plasmon-enhanced spectroscopy and metamaterials development. In this work, we present a systematic study of three aromatic diamines as possible linkers of silver NPs. Symmetric 4,4′-derivatives of bibenzyl, stilbene and tolane have been selected to reveal the effect of linker rigidity and conjugation degree on the morphology and optical properties of the obtained aggregates. Surface-enhanced Raman scattering spectra of colloidal solutions with different aromatic amine additives have been acquired to assess the surface equilibria and adsorbate state. The non-monotonic concentration profiles have been obtained in the case of effective NPs linking and can be considered as its feature. Transmission electron microscopy and absorption spectroscopy have been involved for the analysis of size and morphology of NPs aggregates. Effective linking of silver NPs is established for stilbene and tolane compounds, which give quasi-spherical aggregates and elongated dimers/trimers. Surface-initiated azo conversion is revealed for bibenzyl compound instead of NPs bridging. The tendency to decrease the degree of NP aggregation with increasing aromaticity and “springiness” of molecular linker is observed. The capability of rigid diamines to assemble the nanoparticles with a finite degree of aggregation, even at high concentrations, distinguishes them advantageously from aliphatic bifunctional linkers. Highly stable plasmonic aggregates can be obtained in this way and implemented further as building blocks for nano- and meta-materials, as well as for tags in imaging applications.

Introduction

The controllable aggregation of nanoparticles (NPs) is considered as a helpful process in selected areas of their potential applications. The most striking examples of this are the surface-enhanced Raman scattering (SERS) spectroscopy and spectrophotometry, dealing with plasmonic gold and silver nanoparticles preferably. In SERS spectroscopy, NP aggregates provide the higher enhancement factors compared to single nanoparticles [1], [2], [3], [4], [5], [6]. This is due to the well-known phenomenon of hot spots, consisting in a superposition of local electromagnetic fields in the gap between the closely packed NPs. That is why tSERS substrates based on NP aggregates are used for the most sensitive analyzes, with a detection limit up to single molecules [7], [8], [9]. Spectrophotometry, is based on a change of solution color caused by NPs aggregation, which has been proposed for the detection of many analytes from environmental contaminants [10], [11], [12], [13], [14] and residual drugs [15], [16] to amino acids [17], [18], [19] and proteins [20], [21], [22], [23] in physiological fluids. Progress in colorimetric sensing using noble metal nanoparticles shows high potential due to their simplicity and practicality [24], [25]. Therefore, the development of reproducible and controllable NP aggregation is demanded by many research and application areas.

Functionalization of NP surfaces is often a necessary step to reach the selectivity of their reaction. For instance, silver (Ag) NPs stabilized with citrate and modified by L-cysteine showed sensitive aggregation only in response on mercury (Hg2+) among 16 different metal ions [26]. The role of surface modifier can be both to capture the detected species [27], and ion exchange with them [28]. Among various surface modifiers, bifunctional compounds play a special role, as they can link the nanoparticles in more controllable way [29], [30]. This makes it easier to regulate the interparticle gap, providing hot spots. However, a clear differentiation of conditions of single-end, double-ends and bridge-like adsorption is required for the success of this approach using bifunctional compounds. Understanding the role of the structure of the molecular linker on the morphology and optoelectronic properties of obtained NP aggregates would further expand the use of this approach. Moreover, increasing the range of molecular linkers capable of tuning of the morphology of the NP aggregates is important for the further developments. It should be noted that finding these molecules, which are able to instill new optoelectronic properties simultaneously with NPs linking, is the toughest task.

The most recognized molecular linkers of gold and silver NPs are dithiols [31], [32], [33]. Alternative compounds, include molecular linkers based on amines [34], [35], cyanides [36], thiocarbamate [37], cucurbiturils [38], [39], where amines show the highest affinity to the silver surface. To achieve new optoelectronic properties of hybrid systems, the use of unsaturated molecules is preferable to aliphatic ones. This has been demonstrated in the experiments on the metal clusters junction [40], [41], [42]. In this study, we chose aromatic diamines for the investigation as potential wires for nanoparticles. In the recent work, we already showed that the diamino- derivative of stilbene may link the silver NPs effectively [43]. Moreover, a resonance charge transfer arises between the molecule and nanoparticles in the obtained NPs aggregates [44]. We have systematically expanded the range of aromatic diamines investigated. To the best of our knowledge, prior systematic studies were mainly focused on the influence of linker length on the gap width in NP aggregates [45], [46], [47]. The expected trends of increasing interparticle distance together with increasing linker length were determined. The goal of this work is to reveal the effects of linker rigidity and linker conjugation degree on the morphology and optical properties of the formed NP aggregates. This explains the choice of bibenzyl, stilbene and tolane derivatives represented in Fig. 1.

Section snippets

Materials, chemicals and solutions preparation

4,4′-Diaminobibenzyl (С14H16N2, 95%) (DABB), 4,4′-diaminostilbene dihydrochloride (С14H14N2.2HCl, 95%) (DAS), 4-aminostilbene (AS), also known as 4-[2-phenylethenyl]aniline (С14H13N, 98%), were purchased from Sigma-Aldrich, 4,4′-diaminotolane (С14H12N2, 95%) (DAT) was purchased from Santa Cruz Biotechnology, silver nitrate (AgNO3, 99%), sodium borohydride (NaBH4, 98%), methanol (MeOH, ultrapure grade) were purchased from Vekton (Russia). DABB and DAS were recrystallized in MeOH before use,

Analysis of concentration-dependent SERS spectra of aromatic diamines

The SERS spectra patterns are highly sensitive to the adsorbate state, even minor changes can cause observable band shifts. Moreover, a concentration dependence of the SERS spectra may acontain valuable information about the processes occurring on the surface [43]. Therefore, the SERS spectra of aromatic amines were acquired from a wide range of concentrations (Fig. 2).

DABB showed the greatest difference in signal intensity of the acquired SERS spectra comparing against DAS and DAT,. Its

Conclusions

In the present work, we have investigated the ability of three aromatic diamines to link Ag NPs. It has been shown that linker rigidity and the degree of conjugation are influenced, crucially, by the morphology and SERS response from the modified NPs. Rigid diaminotolane with a central triple bond assembles the nanoparticles into oblong dimers and trimers. Flexible diaminostilbene with a central double bond forms quasi-spherical aggregates, consisting of six to eight NPs. This can be summarized

CRediT authorship contribution statement

Elena V. Solovyeva: Conceptualization, Methodology, Investigation. Aleksei N. Smirnov: Validation. Vasilisa O. Svinko: Investigation. Aleksei S. Strelnikov: Investigation. Alisa I. Shevchuk: Investigation. Sergei G. Kazarian: Conceptualization, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by Saint‐Petersburg State University, project № 92350587, and by RFBR, grant № 20-33-70034. The authors would like to thank the Resource Centers of SPbU: "Optical and Laser Materials Research", "Chemical Analysis and Material Research", "Interdisciplinary Center for Nanotechnology" and “Physical Methods of Surface Investigation”.

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