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Abstract
We developed a simple and rapid method for dimer and higher multimer formation of gold nanoparticles (AuNPs) in bulk suspension. A coupling of AuNPs modified with COOH-terminated alkanethiol by van-der-Waals interaction between alkyl chains was employed. We demonstrated the tunability of the interparticle gap by changing the alkyl chain length from C5 to C15. Efficient dimer formation that avoids unwanted aggregation was demonstrated for AuNPs with a diameter ranging from 20 nm to 80 nm. For all cases, we found that the interparticle gap is well-defined and uniform. For the shortest alkyl chain (C5), we achieved an interparticle gap as small as 1.0 nm.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metal nanoparticles exhibit a unique optical phenomenon known as localized surface plasmon resonance (LSPR), which enables localization and enhancement of an optical field at the nanoscale, and the LSPR wavelength is sensitive to its local dielectric environment. These features are advantageous for employing metal nanoparticles as a solid platform for chemical and biomolecular sensing applications [1–3]. Aggregates or multimers of metal nanoparticles are also attractive because they allow a much higher field enhancement (hot spot) at the interparticle gap, broader LSPR tunability and optical anisotropy depending on the particle configuration, and significant modification of LSPR spectral features induced by Fano interference [4,5]. Both gold and silver are widely used as materials for LSPR in the visible wavelength range. Gold nanoparticles (AuNPs), however, are preferred in terms of stability, size uniformity, and applicability to near-infrared (NIR) light [6–9].
The tunability of the longitudinal LSPR mode of AuNP dimers can be extended into the NIR spectral range (λ >650 nm) by using AuNPs with a small interparticle gap [4,10,11]. This implies that AuNP dimers can be exploited as an alternative to Au nanorods (AuNRs), which are widely used in cellular imaging and drug delivery because of the deeper penetration of NIR light (650~900nm) into biological tissues than visible light for in vivo studies [12–16]. The advantages of AuNP dimers over AuNRs include the presence of an interparticle hot spot, size and shape uniformity, and low toxicity [17]. The uniformity in size and shape of AuNP dimers is a result of that of AuNPs as constituents. The size distribution of AuNPs is much more easily optimized by tuning the growth conditions compared to that of AuNRs. Also, importantly, the growth of AuNPs and the formation of dimers do not involve the highly toxic cetyltrimethylammonium bromide, which is commonly used for the growth of AuNRs as a growth-directing agent.
Engineering of field enhancement at the hot spot is of great interest with respect to surface-enhanced Raman spectroscopy (SERS) because it provides label-free, multiplexing detection of disease biomarkers at a single-molecule level. It is well known that the Raman enhancement factor is critically dependent on the size of the interparticle gap. As a typical case, a AuNP dimer with an interparticle gap smaller than 1 nm gives rise to a Raman enhancement on the order of up to 108 [18–20]. A systematic theoretical calculation of the Raman enhancement factor of AuNP dimers as a function of AuNP size and interparticle gap has been conducted, and it predicts the optimal size and separation to maximize the enhancement factor.
Surface modification of AuNPs with functional molecules can be performed via coordination interaction, electrostatic interaction, van der Waals interaction, hydrophobic interaction and hydrogen bond interaction [21]. Based on these surface modification techniques, a variety of methods for the binding of metal nanoparticles to form multimers in suspension have been proposed and experimentally demonstrated. They include DNA hybridization [22–24], DNA origami [25,26], dithiol coupling and electrostatic interaction via cations/anions [19]. Many of them employ immobilization of metal nanoparticles on solid substrates to avoid unwanted formation of higher multimers, while there have been fewer studies on dimer formation in bulk suspension [27–29]. The immobilization involves tethering metal nanoparticles on substrates by functionalized molecules and also embedding them in nanostructured templates. Although immobilization is useful for SERS substrates and single-dimer spectroscopy, dimers in bulk suspension are more versatile for applications in homogeneous assays. For AuNP dimer formation in bulk suspension, the most important aspect is to obtain as small an interparticle gap as possible to maximize the Raman enhancement and red shift of the longitudinal mode while avoiding excessive aggregation of AuNPs. Regarding AuNP dimers in aqueous suspension, a significantly large red shift of the longitudinal mode, which is a true measure of the interparticle gap and resultant field enhancement factor, has been obtained by electrostatic interaction and also by mediation by rigid molecular linkers.
In this paper, as an alternative simple and rapid method for dimer formation, a coupling of AuNPs modified with COOH-terminated alkanethiol by van-der-Waals interaction between alkyl chains was developed and optimized. We investigated the tunability of the interparticle gap by changing the alkyl chain length and evaluated the shortest interparticle gap. Efficient dimer formation that avoids unwanted aggregation was demonstrated for AuNPs with a diameter ranging from 20 nm to 80 nm.
2. Results and discussion
The surface modification of AuNPs with alkanethiols and their interaction enable formation of AuNP dimers and higher multimers with a controllable interparticle distance. A protocol for multimer formation is illustrated in Fig. 1.
We employed AuNPs with an average diameter of 41 ± 3 nm in solution (pH6-7), which were provided by Tanaka Kikinzoku Kogyo. The AuNPs, coated with bis-p-(sulfonatophenyl)phenyl phosphine (BSPP), were functionalized with alkanethiols having a carboxyl group at the terminus in a modified buffer (0.5 × TBE, 40 mM NaCl, 0.5mg/ml BSPP). To demonstrate the controllability of the AuNP distance, the following four alkanethiols with different alkyl chain lengths (n = 5, 7, 10, 15) were used: 5-carboxy-1-pentanethiol (C5), 7-carboxy-1-heptanethiol (C7), 10-carboxy-1-decanethio (C10) and 15-carboxy-1-pentadecanethiol (C15). Formation of AuNP multimers due to the interaction between the carboxyl alkanethiols was promoted by incubation in a binding buffer with a higher salt concentration (0.5 × TBE, 160 mM NaCl, 0.5mg/ml BSPP) at 92 °C for 5 min.
Figure 2(a) compares the colorimetric response of colloidal suspensions of multimers composed of four different alkanethiol-modified AuNPs (C5, C7, C10 and C15) and unmodified AuNPs (Non), which were also investigated by agarose gel electrophoresis purification, as shown in Fig. 2(b). The clear band separation together with the result of transmission electron microscopy (TEM) shown in Fig. 3(b) demonstrates the formation of well-defined AuNP dimers, trimers and higher multimers with good size uniformity. A distinct difference in the color of the second bands among C5 to C15 indicates the variation in the interparticle distance.
Fig. 2 (a) Suspensions of multimers of 40-nm AuNPs modified with COOH-alkanethiols of C5, C7, C10, C15 and those of unmodified AuNPs (Non). (b) Agarose gels electrophoresis purification of the suspensions in (a).
Fig. 3 (a) Suspensions recovered from the first (monomer: m), the second (dimer: d), and the third (trimer: t) bands in electrophoresis separation of multimers of 40-nm AuNPs modified with C5. (b) TEM images of monomers, dimers, and trimers obtained from the suspensions in (a). (c) Extinction spectra of monomers, dimers, and trimers in the suspensions in (a). (d) FDTD simulation of absorbance spectra of 40-nm AuNP trimers (linear, triangular and their intermediate) in water with interparticle gaps of 1.0 nm.
Figure 3(a) compares the color of the suspensions recovered from the first, second and third bands in the electrophoresis purification for the suspension of AuNP multimers modified with C5. To confirm the homogeneity in multimer formation, Fig. 3(b) shows TEM micrographs of AuNP monomers, dimers and trimers obtained from the suspensions shown in Fig. 3(a). Figure 3(c) shows UV-visible extinction spectra of the monomer, dimer and trimer suspensions of C5-modified AuNPs. The peaks around 530 nm correspond to LSPR on the monomer AuNP and that for the transverse mode of the dimer and trimer. These peaks do not shift even for higher multimers with respect to the peak of the monomer AuNP, and also are independent on the interparticle gap (see Fig. 4(c)). In contrast, the LSPR peak for the longitudinal mode of the trimer undergoes a red shift relative to that of the dimer. The broader band for the trimer is due to the variety of configurations taken on by the three AuNPs. Figure 3(d) shows FDTD simulation of absorbance spectra of 40-nm AuNP trimers with interparticle gaps of 1.0 nm for three different configurations: linear, triangular and their intermediate. From the triangular to the linear configuration, the spectrum continuously shifts to the longer wavelength. Also, the spectrum intensity drastically increases with approaching the straight configuration. Unless the distribution of trimer configuration is biased toward the triangular side, the simulated spectra will reproduce the experimental result.
Fig. 4 (a) Suspensions of 40-nm AuNP dimers recovered from electrophoresis separation for AuNP multimers modified with C5, C7, C10 and C15. (b) Extinction spectra of AuNP dimers in the suspensions in (a). (c) Cryo-TEM images of AuNP dimers obtained from the suspensions in (a). (d) FDTD simulation of absorbance spectra of 40-nm AuNP dimer in water with interparticle gaps of 1.0 nm, 1.4 nm, 2.0 nm and 3.0 nm. (e) Plot of the peak wavelength of longitudinal mode as a function of the interparticle gap obtained by FDTD simulation along with the plot for the experimental results obtained by the cryo-TEM (interparticle gap) and by the absorbance measurement (peak wavelength). (f) Hydrodynamic size distribution of AuNP dimers in the suspensions in (a) measured by dynamic light scattering.
Figure 4(a) shows suspensions of AuNP dimers recovered from the second band in electrophoresis purification of AuNP multimers modified with C5 to C15, and Fig. 4(b) shows extinction spectra of the suspensions. A systematic red shift of the band for the longitudinal mode was observed; the magnitude of the red shift increased in the order of C5 (peak wavelength at λ = 627 nm) > C7 (λ = 608 nm) > C10 (λ = 596 nm) > C15 (λ = 574 nm). Based on the well-known fact that the magnitude of the red shift increases with decreasing interparticle gap [11,21], the result demonstrates that the shorter the alkyl chain length, the smaller the interparticle gap. To confirm this, we directly estimated the interparticle gap by cryo-TEM observation, which allows the natural state of the AuNP dimers to be maintained. The cryo-TEM micrographs shown in Fig. 4(c) indicate that the interparticle gaps of AuNP dimers modified with C5, C7, C10 and C15 are 1.0 ± 0.42 nm, 1.5 ± 0.54 nm, 2.2 ± 0.71 nm and 3.9 ± 0.85 nm, respectively, which were obtained by averaging for ten dimers for each surface modification. These results are consistent with the red shift of the longitudinal-mode band of the AuNP dimer in Fig. 4(b).
Figure 4(d) shows extinction spectra numerically calculated by using a finite-difference time-domain (FDTD) method for 40-nm AuNP dimers with interparticle gaps of 1.0 nm, 1.4 nm, 2.0 nm and 3.0 nm, which are assumed to correspond to the alkyl chain lengths of C5, C7, C10, and C15, respectively (It is reported that the thickness of alkanethiol self-assembled monolayer increases by 0.2 nm with the number of carbon atoms in the alkyl chain [30]). Although the accuracy in measuring the interparticle gap with cryo-TEM is still in debate, simulation and experimental results are in good agreement. In particular, the agreement in the magnitude of redshift of the longitudinal mode with the alkyl chain length demonstrates that the van-der-Waals coupling between alkyl chains is a principal mechanism for the dimer formation, which will be discussed later.
Figure 4(f) shows the results of dynamic light scattering (DLS) measurements carried out to measure the size distribution of the AuNP dimer. The signal due to the translational diffusion of the dimer lies around 64 nm, which is the diameter of a “spherical” particle equivalent to the “dumbbell” dimer with respect to the diffusive motion. The size distributions for C5 to C15 are very similar, and this demonstrates the uniformity of the dimer. The signal around 9 nm originates from the rotational diffusion of the dimer. Since the dimer exhibits distinct optical anisotropy between the longitudinal and transverse modes, rotational motion causes significant fluctuations in the scattered light and correlation peak at a delay corresponding to the rotational diffusion time. The optical anisotropy and the resultant signal intensity are enhanced with a reduction in the interparticle gap (alkyl chain length), which is consistent with the result of a larger red shift of the longitudinal mode with the gap reduction.
We also conducted multimer formation using AuNPs with other diameters, including 20, 30, 50, 60 and 80 nm modified with C5, C7, C10 and C15. The NaCl concentration in the modified buffer and binding buffer was optimized, depending on the size of AuNPs, to maximize the dimer formation efficiency, avoiding unwanted aggregation of AuNPs. Figures 5(a) and 5(b) show the results of electrophoresis separation and extinction spectrum measurement for dimers, respectively. Over the wide range of AuNP size, we confirmed the uniformity of multimers (distinct separation of electrophoresis bands) and a systematic change in the colorimetric response and dimer peak shift with changing alkyl chain length. It should be noted that the degree of the red shift of the longitudinal mode in Fig. 5(b) becomes less significant with a decrease in AuNP size. This is due to the fact that the magnitude of the red shift is sensitive to the relative distance of the AuNPs (the ratio of the interparticle gap to AuNP diameter) as well as its absolute distance. To confirm this fact, Fig. 5(c) shows FDTD simulation of absorbance spectra of 20- to 80-nm AuNP dimers in water with interparticle gaps of 1.0 nm, 1.4 nm, 2.0 nm and 3.0 nm.
Fig. 5 (a) Electrophoresis purification of multimers of AuNPs with a diameter ranging from 20 nm to 80 nm modified with C5, C7, C10 and C15 as well as unmodified. (b) Extinction spectra of AuNP dimers recovered from the second band in electrophoresis purification in (a). (c) FDTD simulation of absorbance spectra of 20 nm to 80 nm AuNP dimer in water with interparticle gaps of 1.0 nm, 1.4 nm, 2.0 nm and 3.0 nm.
Finally, we discuss two possible mechanisms of multimer formation via interaction between COOH-alkanethiols. One is based on hydrogen bonding between COOH termini, which allows COOH-alkanethiols to form a bilayer. The other mechanism is van-der-Waals interaction between alkyl chains, which penetrate into each other. If hydrogen bonding is present and a bilayer is formed, the interparticle gap must be almost equal to twice the length of COOH-alkanethiol, which must be larger than 2.0, 2.8, 4.0 and 6.0 nm for C5, C7, C10 and C15, respectively. However, the interparticle gap estimated by TEM is close to the length of single COOH-alkanethiol, which suggests that the van-der-Waals interaction between alkyl chains is more likely. References [31] and [32] describe metal nanoparticle superlattice structure formed by interdigitating interaction between alkyl chains, estimating the interparticle gap size. In both studies, the alkyl chains might collapse after solvent evaporation. The difference in the terminal group of alkanethiol molecule also have impact on the degree of interdigitation.
To gain a deeper understanding of the binding mechanism, we performed micro-Raman scattering spectroscopy of dimers of 40-nm AuNPs modified with C5 to C15 in suspension (Fig. 6). The observation volume was less than (1 μm)3, which is much smaller than the inverse of the dimer concentration; thus, we measure the Raman spectrum from a single dimer under Brownian motion at a time. The field enhancement at the interparticle gap enables selective enhancement of the Raman signal that originates from interacting COOH-alkanethiols to form a dimer. The shortest interparticle gap (C5) gives rise to the strongest Raman signal, while the signal intensity drastically decreases with the gap distance, which also demonstrates that the Raman signal from the gap region is highlighted. The Raman spectrum for C5 includes peaks assigned to alkanethiol (650 and 757 nm−1) and carboxyl groups (922, 1435 and 1584 nm−1) as well as alkyl chain (980~1200 nm−1) [33–36]. The extinction measurement and TEM observation above indicate that the interparticle distance is well-defined and uniform for all the COOH-terminated alkanethiols. This implies that the COOH-alkyl chains penetrate into the other side at the maximum depth and that the COOH terminus might interact with the Au surface directly and be chemisorbed [37,38].
Fig. 6 Surface-enhanced Raman scattering spectra of 40-nm AuNP dimers modified with C5 to C15, and monomer modified with C5.
We briefly mention a possible application of the SERS measurement scheme above.
In our process for surface modification of AuNP with alkanethiol molecules, the incubation time is only 1 h, which is much shorter than the time required for the formation of self-assembled monolayer. It means that the dimer gap is not fully occupied with alkanethiol molecules and analyte molecules might be sandwiched in the gap volume, which acts as a cage and allows SERS measurements at single-molecule level.
3. Methods
In the following subsections experimental and numerical methods used in this work are described.
3.1 Materials
A colloidal suspension of AuNPs (pH 6-7) was provided by Tanaka Kikinzoku Kogyo. The mean diameter and concentration of 20-nm, 30-nm, 40-nm, 50-nm, 60-nm and 80-nm AuNPs are 20 ± 2 nm and 1.2 × 1012 ml-1, 30 ± 3 nm and 3.7 × 1011 ml-1, 40 ± 3 nm and 1.6 × 1011 ml-1, 50 ± 3 nm and 8.0 × 1010 ml-1, 60 ± 4 nm and 4.7 × 1010 ml-1, and 80 ± 4 nm and 2.0 × 1010 ml-1. Deionized distilled water (DDW) purchased from Millipore (water for molecular biology) was used for all experiments. Bis(p-sulfonatophenyl)phenylphosphine dipotassium salt dehydrate sodium chloride (BSPP), 10 × TBE (0.89 M Tris-borate, 20 mM EDTA: pH 8.2-8.4), ethanol, agarose S, and Ficoll 400 were purchased from Wako Pure Chemical. Alkanethiols 5-carboxy-1-pentanethiol (C387), 7-carboxy-1-heptanethiol (C386), 10-carboxy-1-decanethiol (C385), and 15-carboxy-1-pentadecanethiol (C429) were purchased from Dojindo Molecular Technologies.
3.2 Synthesis of AuNP multimers
The AuNP colloidal suspension was ultrasonically dispersed for 15 min, and 1 ml of the suspension was placed in a 1.5 ml microfuge tube (Eppendorf DNA LoBind Tubes) and mixed with 100 μl of 10 mg/ml BSPP solution prepared using DDW. After incubation at 50 °C for 1 h, the AuNPs were washed by centrifugation for 15 min at 10000 rpm for 20-nm AuNP, 6500 rpm for 30-nm, 5000 rpm for 40-nm, 4000 rpm for 50-nm, 3000 rpm for 60-nm, and 2000 rpm for 80-nm. (Subsequently, the centrifugation process was performed under the same conditions as those listed here.) After washing, the AuNPs were resuspended in 1 ml of modified buffer, which includes 0.5 mg/ml BSPP, 0.5 × TBE and 60 mM NaCl (TBE60) for 20-nm AuNP, TBE50 for 30-nm, TBE40 for 40-nm, TBE30 for 50-nm, TBE20 for 60-nm, TBE20 for 80-nm. The AuNP dispersion was modified with alkanethiol (Non, C5, C7, C10, C15) by adding it with a number ratio of alkanethiol / AuNP = 26,000 for 40-nm AuNP. For the other AuNP sizes, the number ratio was adjusted such that the number of alkanethiol is proportional to the surface area of AuNP. The dispersion was incubated at 50 °C for 1 h. (C5 and C7 were first dissolved with EtOH and then diluted with the same amount of DDW. C385 and C429 were just diluted with the same amount of DDW.) Unbound alkanethiols were removed by centrifugation and the pellet was resuspended with the same modified buffer as above. After two washing cycles, the pellet was resuspended in 500 μl of binding buffer, which includes 0.5 mg/ml BSPP, 0.5 × TBE and 280 mM NaCl (TBE280) for 20-nm AuNP, TBE200 for 30-nm, TBE160 for 40-nm, TBE120 for 50-nm, TBE80 for 60-nm, TBE40 for 80-nm to double the concentration. After denaturation by incubating at 92 °C for 5 min, the suspensions were washed twice by centrifugation with 0.5 × TBE to remove NaCl after returning to room temperature.
3.3 Separation and recovery of AuNP multimers
After centrifugation of the suspension, the pellet obtained was mixed with 1/5 volume of Ficoll400 (100 mg/ml) loading buffer solution and was purified by 1.5% agarose gel electrophoresis (Mini Submarine Gel Electrophoresis Unit from NIHON EIDO. Co. Ltd.) at 175 V for 15 min with 0.5 × TBE as running buffer. The first (monomer), second (dimer) and third (trimer) bands were sliced out of the gel by using recovery system NA-1710 (NIHON EIDO. Co. Ltd.) operated at 100–150 V with 0.5 × TBE as the running buffer. AuNPs suspensions were washed by centrifugation and the pellet was resuspended with 0.5 × TBE.
3.4 AuNPs dispersion of characterization
After centrifugation of 200 μl of suspension, the pellet obtained was mixed with 1/5 volume Ficoll400 (100 mg/ml) loading buffer solution and purified by 1.5% agarose gel electrophoresis operated at 175 V for 15 min with 0.5 × TBE as the running buffer. The bands in the gel were photographed using a digital camera (Canon PowerShot G7X). AuNPs recovered from the first, second, and third bands were observed using transmission electron microscopy (TEM: TECNAI G2, TECNAI Sprits from FEI). UV-visible absorption spectroscopy (UV-Vis: BioSpectrometer from Eppendorf) was used to characterize the optical extinction (absorption and scattering) due to the localized surface plasmon resonance. The inter-particle distance of the dimers in their natural state was evaluated by cryo-TEM (Titan Krios from FEI). The size distribution of the AuNP multimers was measured by dynamic light scattering using a Nano ZS zetasizer system, which employs a He-Ne laser with a wavelength of 633 nm, at 25°C. Raman scattering spectra were acquired using InVia Raman spectroscopy system (Renishaw Plc.) equipped with a He-Ne laser as an excitation source. A laser beam of 8 mW was focused to a spot in the dimer suspension with a liquid sample using a microscope objective of NA 0.5. Raman spectra obtained in a 1 sec acquisition time for each measurement were accumulated 200 times.
3.5 Numerical simulation
A 3D electromagnetic field simulation was performed by using Lumerical FDTD solutions. Absorbance (scattering and absorption) spectra for 40-nm AuNP dimer in water were calculated for various interparticle gaps. A circularly polarized plane wave source was used to simulate a random distribution of the dimer orientations in water. The dielectric function of gold was adopted from the experimental data obtained by Johnson and Christy [39]. A uniform mesh of cubic cells, 0.5 nm × 0.5 nm × 0.5 nm, was used.
4. Conclusion
We developed and optimized a method for dimer formation of AuNPs modified with a COOH-terminated alkanethiol layer by employing van-der-Waals interactions between the alkyl chains in a bulk suspension. We demonstrated the tunability of the interparticle gap by changing the alkyl chain length. For all cases, we found that the gap is well-defined and uniform. This implies that the interaction between COOH, a terminus of alkanethiol, and the Au surface also play a role in dimer formation. For the shortest alkyl chain, we achieved an interparticle gap as small as 1.0 nm.
Funding
Ministry of Education, Culture, Sports, Science and Technology (16H03889, 18H04490); Advanced Photon Science Alliance Project from MEXT, Osaka University.
Acknowledgments
We are grateful to Tanaka Kikinzoku Kogyo for providing gold nanoparticles for this study.
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