# Co-Solvent-Aided Opto-Thermophoretic Printing of Gold Nanorod Assemblies

Ana Jiménez Amaya, Claire Goldmann, Maike C Haidasch, Cyrille Hamon, Eric H Hill

► To cite this version:

Ana Jiménez Amaya, Claire Goldmann, Maike C Haidasch, Cyrille Hamon, Eric H Hill. Co-Solvent-Aided Opto-Thermophoretic Printing of Gold Nanorod Assemblies. Advanced Optical Materials, In press, 10.1002/adom.202500375. [https://doi.org/10.1002/adom.20252]

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# Co-Solvent-Aided Opto-Thermophoretic Printing of Gold Nanorod Assemblies

Ana Jiménez Amaya, Claire Goldmann, Maike C. Haidasch, Cyrille Hamon, and Eric H. Hill*

The controlled deposition of plasmonic nanoparticles on solid surfaces is critical for the fabrication of medical diagnostics, devices, and sensors. Despite recent advances in harnessing thermal gradients to achieve controlled deposition of nanoparticles at a laser spot, the deposition of anisotropic gold nanoparticles by this approach is a challenge in aqueous solution. Herein, the use of ethanol as a co-solvent is shown to enable rapid (<10 s) and effective deposition of gold nanorods in the presence of polyvinylpyrrolidone on unmodified glass substrates. The influence of key parameters, including polymer concentration and molecular weight, co-solvent content, nanoparticle concentration, and laser power, is studied. While ethanol content and polymer size have the largest impact on the assembly structure, all have an impact on the assembly growth rate. Polymer size is found to significantly impact the degree of nanorod assembly found in the pattern, which leads to a surface-enhanced Raman scattering enhancement factor up to 10⁸. In summary, this approach utilizes thermal gradients to direct the formation of highly localized plasmonic assemblies without the need of functionalization of the substrate. This simple, fast, and adaptable approach is a promising platform for the scalable patterning of plasmonic nanostructures.

## 1. Introduction

Noble metal nanoparticles (NPs) are of great interest due to their unique optical properties, which are directly related to localized

surface plasmon resonance (LSPR) in the visible to near-infrared region of the electromagnetic spectrum, making them potential candidates for a wide number of applications, especially those involving anulyte detection by surface-enhanced Raman scattering (SERS), photothermal therapy, and photocatalysis. Among plasmonic noble metal NPs, gold nanorods (AuNRs) are anisotropic nanostructures whose LSPR can be tuned through their aspect ratio. AuNRs also have the propensity to assemble into ordered superstructures, granting collective plasmonic properties which are advantageous for a number of applications (particularly SERS spectroscopy), driving the development of techniques for NP as assembly at interfaces.[1] The synthesis of NPs of varying size, shape, composition, and surface chemistry has been well-established due to scientific progress in colloidal synthesis via wet-chemical methods in the last decades.[2] However, the capability to assemble such NPs into well-defined structures requires careful balance of their interactions.[2-4]

Tuning of the colloidal environment can allow access to dif-ferent lattice symmetries,[5-8] particle orientations,[9] and even enable complex NP superlattices.[10,11] Template-assisted self-assembly methods based on capillary forces encountered at interfaces during evaporation-induced self-assembly have shown great promise for the fabrication of ordered assemblies of NPs on surfaces.[12-14] However, the requirement of cleanroom tech-niques to produce the silicon masters used for the templates lim-its the capability for free-form patterning of plasmonic NP as-sembles on solid substrates, in addition to the drawbacks of liter-scale NP synthesis and deposition times up to hours.

Recently, directed approaches which are driven by light or ex-ternal electric, magnetic, and acoustic fields have emerged as ver-satile solutions to achieve the assembly of NPs at specific regions on surfaces.[13,15] Optically-directed assembly of gold nanopar-ticles (AuNPs) offers the capability to fabricate plasmonic sub-strates on-demand with tailorable structure. AuNPs are capable of heating under irradiation through plasmon-enhanced photon-phonon conversion,[16] which can be used to form a tempera-ture gradient in liquids. The manipulation of a wide number of species is possible due to their different response to this gradi-ent, directing them toward or outward the target area. The Soret

A. Jiménez Amaya, M. C. Haidasch, E. H. Hill
Institute of Physical Chemistry
University of Hamburg
Grindelallee 117, 20146 Hamburg, Germany
E-mail: eric.hill@chemie.uni-hamburg.de

A. Jiménez Amaya, E. H. Hill
The Hamburg Center for Ultrafast Imaging (CUI)
Luruper Chaussee 149, 22761 Hamburg, Germany
C. Goldmann, C. Hamon
CNRS, Laboratoire de Physique des Solides
Université Paris-Saclay
Orsay 91405, France

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The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202500375
© 2025 The Author(s). Advanced Optical Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adom.202500375
effect, also termed thermophoresis, describes the motion of particles within a liquid mixture when subjected to a thermal gradient. Under a thermal gradient, a solute of concentration c contained in a liquid has a flux density defined as

$$J(r,t) = -D\nabla c(r,t) - c(r,t) D_T \nabla T(r,t) \quad (1)$$

where J is the flux density, D and D_T are the mass and thermal diffusion coefficient, respectively and ∇T is the temperature gradient. The first term describes the contribution to solute flux due to the concentration gradient (normal mass diffusion behavior). The second term accounts for the additional flux due to thermal diffusion. By applying a steady-state approximation, the term J equals zero, and Equation (1) turns into:

$$\nabla c = -c \cdot S_T \nabla T \quad (2)$$

where S_T stands for the Soret coefficient. S_T is known as the ratio of heat transport to mass transport:

$$S_T = \frac{D_T}{D} \quad (3)$$

Depending on the sign of S_T, the thermophoretic behavior of a colloid will be thermophobic (positive S_T) or thermophilic (negative S_T). This coefficient is typically on the order of S_T ≈ 0.01–10 K⁻¹ depending on the object size and other properties, such as solute-solvent interactions, salinity, solute concentration, surface properties, temperature, etc.[17–19] Unfortunately, S_T is not easily predicted and has only been measured for a limited number of colloids.[20–22]

Plasmonic heating of AuNPs with a laser has been previously shown to provide a thermal gradient,[23] which can also drive the thermophoresis of AuNPs toward the laser spot, dubbed optothermophoresis. Earlier studies of this phenomenon used plasmonic substrates in order to provide heating at the laser spot, which enabled the reversible assembly of AuNPs and other colloids.[24–27] However, printing of colloidal particles onto substrates has presented a challenge, and has only been achieved under specific conditions. Printing of polystyrene and silica microspheres was demonstrated by their trapping in polymer hydrogels and subsequent UV-induced crosslinking, however, this method relies on a plasmonic substrate, limiting the flexibility of the approach.[28] On the other hand, the occasional printing of trapped gold nanoparticles onto plasmonic substrates has been reported during prolonged trapping experiments (>20 min).[26]

We recently reported the laser-directed opto-thermophoretic deposition of gold nanospheres from an aqueous dispersion onto glass substrates,[29] however, this approach could only function under a limited range of colloidal conditions, which could not be transferred to anisotropic particles such as AuNRs. Herein, the use of EtOH as a co-solvent is shown to enable opto-thermophoretic printing of anisotropic NPs. While optothermophoretic printing of AuNRs was not possible in aqueous solution, the addition of ethanol as a co-solvent not only enabled the printing of AuNRs of different size, but also greatly enhanced the speed of the process. Furthermore, the size and concentration of the polymer polyvinylpyrrolidone (PVP) was found to significantly impact both the speed of the assembly and the degree of NP assembly, which significantly impacted the SERS performance of the resulting micropatterns.

Overall, this not only enables rapid optically-controlled micropatterning of anisotropic metal NPs onto substrates but also reveals the importance of solvent composition in tuning the overall process of thermophoresis and adhesion to the substrate interface. This opens the door to further advances in optically-directed patterning of plasmonic substrates and nanofabrication in general.

# 2. Results and Discussion

## 2.1. Influence of Solvent Composition on AuNR Printing

The optothermophoretic printing of AuNRs was attempted following a similar approach as in our previous work with gold nanospheres (AuNS)[29] to study their assembly formation. Surprisingly, under the same colloidal conditions (i.e., [PVP10K] 0.1–10 mg mL⁻¹, [Cetyltrimethylammonium chloride (CTAC)] 0.04–0.08 mM, [Au⁰] 5 mM) the assembly and printing of AuNRs in aqueous solution was not observed, even when [Au⁰] was increased to 15 mM. It is well known that increased curvature of the gold nanoparticle surface results in less dense coverage of surfactant, i.e., the density of grafted ligands is greater on the faces of AuNRs than in AuNS.[34] In the previous study, it was noted that an “excess” in the concentration of surfactant (≈0.1 mM CTAC in the case of AuNS) inhibited the formation of assemblies by preventing the surface adsorption by PVP. This suggests that the higher CTAC density on the surfaces of rods may inhibit their assembly in aqueous solutions.

To explore this hypothesis, various amounts of ethanol were added to the dispersion. The presence of ethanol has a strong influence on the critical micelle concentration (CMC) of CTAC, increasing it from ≈1.1 mM in aqueous media to ≈125 mM in equimolar mixtures of H₂O/EtOH.[35] Furthermore, a previous study combining experiment and molecular simulation showed that the presence of ethanol as a co-solvent favored the assembly of AuNRs due to the stripping of interfacial surfactant micelles, thus decreasing inter-particle repulsion.[36] Thus, the mechanism of particle attachment to the surface appears to be the polymer-induced destabilization of the nanoparticles in the vicinity of the thermal gradient, which is inaccessible without the presence of co-solvent due to the increased density of CTA⁺ adsorbed at the nanoparticle faces (Figure 1).

Beyond the effect of co-solvent on the CMC of CTAC, many studies have shown sign changes in the Soret coefficient of several polymers when introducing a co-solvent. Polyethylene glycol (PEG) is a widely studied non-ionic polymer in the context of thermophoresis.[37–39] Its migration under a thermal gradient in aqueous solution is typically thermophobic (S_T is positive). However, a sign change of S_T upon addition of EtOH has been reported.[38,39] When increasing EtOH content in a poor solvent for PEG, the mass diffusion coefficient D increases due to the decreased viscosity of the solvent and decreased hydrodynamic radius of the polymer. This increase of D translates into a decrease of the Soret coefficient according to Equation (3), and therefore, more thermophilic behavior. With this in mind, it has also been shown that EtOH also decreases the radius of PVP in a similar way, which implies that PVP may undergo similarly increased thermophilic behavior.[40] This mechanism supported by a drastic increase in both assembly kinetics and incubation time at
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Working principle of the thermophoresis-induced assembly of AuNRs. a) Schematic of the optical set-up used for printing the patterns. b) Dispersion of AuNRs prior to laser-induced heating, with zoom showing how EtOH strips CTAC from the interface. c) Thermophoretic migration of AuNRs and PVP coils to the hot region (laser spot) once the heating is applied. d) Irreversible assembly of AuNRs on the substrate by PVP. Yellow arrows indicate the direction of the thermal gradient.</figcaption></figure>

14 mW and 1 mg/mL PVP10K when printing AuNS (5 mM Au⁰) in 20% EtOH (4.74s ± 1.02 and 0.302 µm s⁻¹) versus pure aqueous solution (50.27s ± 2.77 and 0.098 µm s⁻¹).

The influence of the volume fraction of EtOH on the kinetics of growth, incubation time, and packing and arrangement of the assembly formed was studied. The working range was studied from 10% to 20% EtOH to ensure the stability of the dispersion (above 30% significant NP aggregation was observed). With an

AuNR concentration of 15 mM Au⁰ and 1 mg mL⁻¹ PVP10K concentration, the incubation time (i.e, the time between when the laser is turned on and when the assembly starts to form around it, t = 0s) decreases and the radial growth rate increases upon addition of EtOH (Figure 2a,b). After an initially linear growth function, the radial growth rate slows, and the overall rate follows an exponential function. This can be attributed to the competition between thermal diffusion and Brownian motion, as

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Kinetics of the assembly growth with increasing volume fraction of EtOH (1 mg mL⁻¹ PVP10K and 14 mW of laser power). a) Radial growth rate in the first 5 s. b) Incubation times. c) Snapshots of the assembly growth, the green circle represents the laser spot. Time t = 0 corresponds to the time when assembly around the laser spot begins to be observed.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Assembly of L-AuNRs at different EtOH volume fraction (10, 15, and 20%v/v). PVP10K concentration is fixed to 1 mg mL⁻¹. Time of growth of each assembly is 10 s, and the laser power used is 14 mW. a–c) corresponds to the assembly with 10, 15, and 20% EtOH, respectively.</figcaption></figure>

described by Equation (1). Initially, the thermal gradient $\Delta T$ drives the NPs toward the laser spot, with radial growth following a linear trend. However, as the NPs assemble around it, local depletion of NPs from the surrounding medium occurs, reducing $c$ and consequently lowering $J$. This leads to the mentioned transition from linear to exponential behavior, indicating the shift from a thermophoretic-driven transport (out-of-equilibrium) to a Brownian diffusion-driven process that tends to homogenize the dispersion. At higher concentrations (EtOH, AuNRs, or PVP), this transition occurs more rapidly due to the more intense initial flux $J$, explaining why the exponential behavior emerges earlier in such cases.

The packing and arrangement of the NPs is also strongly affected by the increasing ethanol concentration (Figure 3). At the same laser power and growth time (10 s), the assemblies appear more densely packed with increasing ethanol. The stark difference between assemblies printed at 10% EtOH and those at 15 and 20% is influenced by both the difference in growth rate, which leads to more particles by 10 s, and the influence of solvent on interfacial interactions between nanoparticles. EtOH, as a less polar solvent than water, would screen the charges of the $CTA^+$ modified AuNR surface less effectively. Furthermore, it was previously shown that the addition of ethanol to a concentrated aqueous dispersion of AuNRs could trigger their aggregation and assembly due to the stripping away of CTAC micelles from the nanoparticle interface[36] and decreased screening of electrostatic repulsion due to reduced solvent polarity. This can not only accelerate the process of aggregation with PVP but also

lead to increased AuNR assembly. This could be further affected by the change in conformation of the polymer coils. From 10 to 20%(v/v) EtOH, the polymer radius decreases, and the conformation takes on a more globule-like structure.[40] There is no evidence of AuNR aggregation prior to printing, as non-specifically adsorbed AuNRs away from the printed assembly appear non-aggregated (Figure 3), supporting a printing mechanism driven by PVP-mediated interactions between AuNRs and the glass substrate.

The influence of particle size was also studied. Using S-AuNRs of 16 nm in width and 60 nm in length, the effect of the solvent composition was examined (Figure S4a,b, Supporting Information). By adding 10%(v/v) EtOH, assembly formation was observed. No relevant differences with larger rods were noted. As the EtOH content was increased to 20%(v/v), increased packing of the assembly was observed, similar to the behavior of L-AuNRs (Figure 3).

## 2.2. Influence of Polymer Molecular Weight and Concentration

To further investigate the working theory that the assembly process is induced by a PVP-mediated destabilization of the AuNPs, the effect of the polymer molecular weight was studied. PVP of 10, 55, and 360 kDa (1 mg mL⁻¹) was used with a fixed EtOH amount of 20%(v/v) and laser power of 14 mW. An increase of the incubation time was observed as the molecular weight of PVP increased, although the assembly growth rate was not
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Kinetics of the assembly growth with varying PVP M.W. (1 mg mL⁻¹ PVP, 20%EtOH, and 14 mW of laser power). a) Radial growth rate in the first 5 s, b) Incubation times, c) Snapshots from the tracking of the assembly growth. Green circle represents the laser spot. Videos of the printing process under these and other conditions are provided in the Supporting Information (Videos S1–S3, Supporting Information).</figcaption></figure>

significantly affected (Figure 4a,b). By working in a more dilute polymer regime (i.e, decreasing [PVP] to 0.1 mg/mL), a drastic change was observed, where the incubation time decreased as PVP size increased (4.68s ± 0.45 > 2.97s ± 0.50 > 2.77s ± 0.37 for PVP10, 55, and 360 K, respectively). In our previous work, the reverse effect was observed with increasing MW when using AuNS in aqueous solution under similar conditions.[29] Since the optothermophoretic printing of AuNRs in pure water could not be achieved thus far, experiments using AuNS with 20%(v/v) EtOH were carried out in order to elucidate whether this phenomenon is due to the role of solvent or NP shape anisotropy.

In pure water, larger PVP led to longer incubation times regardless of the concentration used, whereas adding 20%(v/v) EtOH reversed this trend, shortening incubation times with increasing PVP MW. This can be attributed to the increased thermophobicity of larger PVP molecules in water.[41] However, in EtOH/H₂O mixtures, solvent-induced polymer coil conformations play a stronger role at higher MWs, as observed in poly-9,9-dioctylfluorene (PFO),[42] where higher MW polymers show a larger fraction of condensed structures in the presence of a poor solvent and would explain a decrease in S<sub>T</sub> (more thermophilic). Consequently, incubation time differences at fixed conditions while varying solvent composition are more pronounced for PVP360K than for PVP10K. For example, with a laser power of 9.6 mW and 5 mM Au⁰, incubation times of 5.43s ± 0.99 and 10.97s ± 2.15 for PVP360K and PVP10K (1 mg mL⁻¹) in 20%(v/v) EtOH were obtained, respectively. On the other hand, incubation times of >300s for PVP360K and 128.33s ± 13.01 s for PVP10K were achieved in aqueous dispersions under the same conditions.

The influence of PVP10K concentration with a fixed EtOH concentration of 20%(v/v) was also studied. With increasing polymer concentration, the incubation time decreased from 4.69s ± 0.45 at 0.1 mg mL⁻¹ to 2.4s ± 0.27 at 1 mg mL⁻¹. The increase in

polymer concentration likely results in a faster ligand exchange, allowing the polymer to functionalize both gold and glass surface around the hot spot in less time. As the incubation time decreases, the radial growth rate shows a slight increase with increasing polymer concentration (Figure S2e,f, Supporting Information). Additionally, the concentration range in which printing is observed narrows as PVP MW increases. While printing was observed between 0.1–10 mg mL⁻¹ for PVP10K, it became unfeasible above 1 mg mL⁻¹ for PVP360K. This behavior was also observed for AuNS, likely due to stronger steric repulsion arising from larger polymers at high concentrations, with a lower threshold limit for higher MW PVP.

Comparing the appearance of the assemblies by scanning electron microscopy (SEM), the assembly printed with 1 mg mL⁻¹ PVP (Figure 5b) is more packed that the one with 0.1 mg mL⁻¹ PVP (Figure 5a). This suggests a positive correlation between the number of polymer coils grafting the gold surface and the close packing of the NPs. The effect of the laser power was also analyzed by decreasing from 14 to 8.6 mW. While a lower growth rate and increasing incubation time (Figure S2c,d, Supporting Information) as well as a smaller size of the pattern at a fixed deposition time (Figure S3, Supporting Information) was observed upon decreasing the power (explained by a less intense temperature gradient) there were otherwise no notable differences. In contrast, there is a noticeable difference when comparing the patterns printed with PVP10K and PVP360K (Figure 5b,c). While PVP10K led to more random packing in the center area, PVP360K induced side-by-side assembled nanorods, resulting in small domains of assembled monolayers several micrometers along the pattern.

At a fixed concentration (1 mg mL⁻¹), PVP360K, due to its larger size, adsorbs onto the metal NP surfaces more effectively, promoting more organized assemblies which are energetically favorable (tip-to-tip, side-to-side).[43] In contrast, the smaller
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Scanning electron microscopy images of printed assemblies of L-AuNRs at different PVP concentrations and molecular weights, with a laser power of 14 mW and EtOH volume fraction of 20%. The time of growth of each assembly is 10 s. a) 0.1 mg mL⁻¹ PVP10K; b) 1 mg mL⁻¹ PVP10K; c) 1 mg mL⁻¹ PVP360K.</figcaption></figure>

PVP10K has weaker, shorter-range interactions with the NP surface, leading to less assembly and more random orientations. Similar results for small nanorods (S-AuNR) were obtained when comparing the effect of PVP molecular weight: more side-to-side assembly in the center region of the ring with larger polymer size (Figure S4b,c, Supporting Information). This supports a strong polymer size dependence on the resulting nanostructure of the printed assembly, regardless of the size of the NP.

## 2.3. Mechanism of Optothermophoretic Assembly and Printing of AuNRs

As discussed in the methods section, the AuNR dispersion is iluminated with a focused 532 nm laser. Once the laser is on, the absorption of light by the AuNRs creates a thermal gradient pointing toward the hot region. According to their thermophoretic mobility $D_T$, species in the solution will be driven to hotter or colder areas. Polyvinylpyrrolidone coils, in the presence of ethanol, are directed toward the hot spot along with the NPs (Figure 1b), while other species in the solution, such as the mentioned ethanol and CTA⁺ molecules migrate away from the laser spot. In the hot zone, PVP coils, due to their strong binding affinity for the gold,[44] adsorb onto the exposed areas of the surface (CTAC concentration is kept below the CMC 1.3 mM),[45] leading to the irreversible assembly and deposition of the AuNRs at the substrate surface.

Although thermophoresis in gases is widely studied and established, thermal diffusion in liquids remains a complex and less predictable phenomenon.[46] Colloid-macromolecule mixtures in liquids have been studied in the context of thermophoresis, and

previous studies have shown that it is possible to achieve trapping of colloidal particles in the hot region through depletion attraction.[26,47–49] This phenomenon can be further understood through the Asakura–Oosawa (A–O) theory,[50] which provides a fundamental framework for the understanding of depletion-induced assembly of colloids. According to this theory, depletion interactions arise in systems where non-adsorbing polymer coils are excluded from the vicinity of the particle surface, creating an osmotic pressure imbalance that leads particle aggregation. However, depletion-induced colloidal assembly typically requires weak or negligible interactions between polymer and colloid, allowing the polymer to remain in solution rather than adsorbing onto the particle surfaces. In contrast, PVP is known to have a strong affinity for noble metal and silica surfaces, leading to significant adsorption rather than polymer exclusion.[51,52] This results in so-called bridging flocculation, where polymer coils adsorb onto multiple particle surfaces, promoting assembly.[53,54]

As previously mentioned, at concentrations of CTAC above 0.1 mM, ≈10 times below its CMC, printing was no longer observed. This observation contradicts a depletion-based mechanism, where NP assembly should occur as long as the depletant concentration (in this case, PVP) is sufficiently high to create the necessary osmotic imbalance to overcome charge stabilization. Instead, these findings support that the interactions between PVP, gold and glass surfaces play a fundamental role in the process, leading to significant adsorption rather than polymer exclusion from the particle surface. Polyvinylpyrrolidone is known to bond with noble metal surfaces through the carbonyl group of its pyrrolidone units, and recent work by Fan et al. provides further evidence of this affinity through DFT calculations.[44] Their study demonstrated that PVP binds more strongly to Au than CTA⁺,
<figure><img src="image_9.png" /><figcaption>**Figure 6.** SERS of CV on the printed patterns at 785 nm. a) Raman spectra in different EtOH volume fraction, c) Average Raman Intensity at 1620 cm⁻¹ under different conditions, b) and d) optical images (left) of the printed assemblies and their corresponding SERS mapping (right) for PVP10K and PVP360K, respectively. Scale bar = 5 µm. The outline in (b,d) indicates the area of mapping.</figcaption></figure>

with reported binding energies of −1.07 eV for PVP and −0.97 eV for CTA+. This difference enabled ligand exchange when CTA+-capped AuNPs were exposed to a highly concentrated PVP dispersion in ethanol, a process analogous to the conditions in our system. This scenario is consistent with the previously mentioned bridging flocculation, where polymer chains interact with both gold and glass surface simultaneously, promoting aggregation. Furthermore, the suppression of assembly at high PVP360K concentrations (>1 mg mL⁻¹) supports the hypothesis that surface adsorption as the primary factor governing the process. This inhibition is likely due to steric stabilization from a dense polymer layer on the AuNR surface, a well-documented transition from bridging-induced aggregation to repulsion in polymer adsorption studies.[55]

## 2.4. Surface-Enhanced Raman Scattering

The influence of solvent, polymer concentration, and molecular weight on the SERS activity of crystal violet (CV) was measured based on the maximum intensity of the peak corresponding to the C-C ring stretching mode (1620 cm⁻¹). It should be noted that although there are no significant differences in maximum SERS intensity within a printed assembly, the SERS intensity distribution is more homogeneous in the patterns printed with PVP360K than with PVP10K (Figure 6b,d). Interestingly, the areas near the center of the pattern, which are only sparsely-filled with AuNRs

have similar SERS intensity as the outer rim (Figure 6d). No significant differences were observed by tuning the EtOH amount, PVP10K concentration, or PVP molecular weight. Compared to SERS of similar AuNRs in which CV was used as the analyte, the enhancement factors obtained (EF ≈10⁷−10⁸) were up to 4 orders of magnitude higher than the values reported by Smitha et al using 785 nm excitation.[57] On the other hand, compared to the Surface Enhanced Resonance Raman Spectroscopy (SERRS, excited at 633 nm) of CV in AuNR supercrystals, up to an order of magnitude increased EF was observed.[56] Although the measured EF values are relevant for analytical studies,[58,59] the reproducibility of the signals across the substrate could be improved by better control over the nanogap distance between AuNRs, which is an interesting avenue for future opto-thermophoretic printing approaches.

## 3. Conclusion

Herein, a simple and versatile technique was developed for the patterning of gold nanorod assemblies on glass substrates by using a laser-induced thermal gradient. Our findings demonstrate that the presence of a co-solvent, in this case, ethanol, is essential for achieving the printing of assemblies onto the substrate. Without co-solvent, AuNRs remain dispersed rather than undergoing controlled deposition, highlighting the critical role of solvent composition. The influence of key parameters, such as ethanol content, polymer concentration, and molecular weight, on the
radial growth rate and the spatial arrangement of the nanorods was studied, showing that the ethanol content and the polymer molecular weight strongly influenced the final pattern. While increasing EtOH content results in an assembly with more defined edges and faster growth rate, the polymer MW has a significant impact on the spatial arrangement in the center area of the assembly, increasing the side-by-side and tip-to-tip assembly of AuNRs with the largest polymer PVP360K. However, these effects were shown to be independent of the AuNR size, indicating that the underlying mechanism in not governed by size-dependent interactions. Notably, we identify a limiting factor, the CTAC concentration, where above ≈0.1 mm, the assembly is suppressed, and no printing is observed. This threshold reinforces the fundamental role of PVP in destabilizing the gold surface by competing with CTA⁺ molecules for real estate on the AuNR surface. Such a mechanism is also consistent with recent findings by Pu et al. in which the thermophoresis of silica particles was governed by the adsorption and desorption of surfactants at the particle surface.[60] Moreover, we measured the SERS activity of the analyte crystal violet, obtaining promising enhancement factors (10⁷⁻¹⁰⁸), thus showing the potential of this approach in the micropatterning of SERS substrates.

This method offers important advantages due to its simplicity, speed, and no requirement of substrate functionalization. By tuning the solvent composition and polymer properties, the assembly process can be easily controlled, showing the versatility of this technique for advanced patterning, nanofabrication and optically-directed assembly. Future work should focus on the optimization of this technique for general applicability to a wide range of colloidal systems, such as nano- and micro-sized particles of different sizes, shapes and composition. For instance, nanoparticles composed of materials with lower thermal conductivity could lead to increased deposition rates, although this may necessitate the use of plasmonic substrates, unlike the present study in which gold nanoparticles enabled both photothermal heating and assembly without additional surface modifications. Furthermore, it is of vital importance to gain a better understanding of how NP surface chemistry and colloidal environment influences the balance of interparticle forces in order to address the current limiting factors, such as the working range of [CTAC]. Overcoming these issues will enhance the potential of this technique as a platform for the optical patterning of structures of diverse functionalities and applications in emerging fields such as sensing, imaging, and biomedicine, where rapid and scalable assembly strategies are increasingly in demand.

## 4. Experimental Section

**Materials:** Polyvinylpyrrolidone (PVP, M~w~ = 10, 55 and 360 kDa, dubbed PVP10K, PVP55K, and PVP360K herein), cetyltrimethylammonium chloride (CTAC, 25 wt% in H₂O), crystal violet (CV, >90%), gold chloride trihydrate (>99.9%), NaBH₄ (>96%) and ascorbic acid (>99%) were purchased from Sigma-Aldrich. Ethanol (EtOH, technical grade) was obtained from LLG Labware. Water with a resistivity of 18.2 MΩ-cm was used in all experiments. Glass cover slips (Thickness 1, Sigma-Aldrich) were used for printing.

**Gold Nanorod Synthesis:** Small AuNRs (S-AuNRs, 16*60 nm, AR = 3.8) were synthesized following a previously-described approach.[30] **Seeds:** Under very fast stirring at 30 °C, 300 µL of NaBH₄ (10 mM) were added to a solution of 50 µL of HAuCl₄ (25 mM) and 4.7 mL of cetyltrimethylam-

monium bromide (CTAB) (100 mM). This solution was kept under slow stirring for 1 h before use. **Growth:** 450 mg of bromosalicylic acid was dissolved under sonication in 49 mL of CTAB (50 mM). Then, under stirring at 30 °C, 476 µL of AgNO₃ (100 mM) and 10 mL of HAuCl₄ (25 mM) were added. The reaction was followed by UV-visible spectroscopy in the 300–500 nm wavelength range with a 10 mm path length. When the absorbance at λ~max~ reached 0.7, 1250 µL of ascorbic acid (100 mM) were added, followed by 800 µL of the previous seeds. After 4 h at 30 °C, the nanorods were centrifuged for 10 min at 10 000 rpm and washed twice with CTAC (1 mM).

Large AuNRs (L-AuNRs, 28*150 nm, AR = 5.4) were synthesized following a previously-described approach.[31] **Seeds.** Under very fast stirring at 30 °C, 460 µL of NaBH₄ (10 mM)/NaOH (10 mM) were added to a solution of 200 µL of HAuCl₄ (25 mM) and 9.8 mL of CTAB (100 mM). This solution was kept under slow stirring for 1.5 h before use. **Growth.** At 30 °C, to 390 mL of CTAB (100 mM), 8 mL of HAuCl₄ (25 mM), 2.8 mL of AgNO₃ (100 mM), and 20 mL of hydroquinone (100 mM) were added. After 1 min, 6.4 mL of seeds were added to this clear yellow solution, and the reaction was kept under slow stirring at 30 °C overnight. The nanorods were centrifuged at 5000 rpm for 15 min and washed twice with CTAC (1 mM). Transmission electron microscopy images and UV-vis spectra of both batches of AuNRs are shown in Figure S1 (Supporting Information).

**Opto-Thermophoretic Printing of Gold Nanorods:** All experiments were carried out using an optical setup consisting of a 532 nm continuous wave (CW) laser (Torus, Laser Quantum, Stockport, U.K.). The laser beam was directed through a 5x beam expander (GBE05-A, Thorlabs, Newton, NJ) into an inverted microscope (Eclipse Ti2-A, Nikon, Tokyo, Japan). The beam was directed through a 532 nm dichroic beam splitter and focused onto the sample by illuminating the back of a 60x air objective (numerical aperture (NA) = 0.85, CFI Plan Fluor, Nikon) resulting in a spot size of ≈0.7 µm. Under transmitted light illumination, the image was reflected back to a CCD camera via a notch filter (NF533-17, Thorlabs, Newton, NJ), which blocks the laser light from the camera. The positioning of the beam was managed using a motorized microscope stage (H117P1NN, Prior Scientific Instruments, Cambridge, U.K.).

Dispersions of 15 µL were prepared with AuNRs concentrations from 5 to 15 mM Au⁰ and 0.08 mM CTAC for all experiments. An imaging spacer of 0.12 mm in diameter (SecureSeal, Grace Bio-Labs, Bend, OR) was adhered onto glass cover slip without any previous treatment and the 15 µL dispersion was pipetted into the circular well. Another glass cover slip was placed on top, and a slight pressure was applied to ensure adhesion and push out excess liquid. The influence of ethanol on the assembly process could be studied by varying solvent composition up to 20% (v/v) with fixed PVP concentration. In addition, the influence of polymer concentration (from 0.1 to 1 mg mL⁻¹) and laser power (8.6 to 14 mW) were studied. To determine the effect of polymer size, PVP with molecular weights of 10, 55, and 360 kDa were studied with a fixed concentration (1 mg mL⁻¹) in 20% EtOH. For comparison with AuNRs, the printing of gold nanospheres (AuNS – diameter = 23 nm) in 20% EtOH was also carried out similar to the previous work.[29]

**Scanning Electron Microscopy:** The printed assemblies were characterized by scanning electron microscopy (SEM) (Zeiss LEO Gemini 1550, Zeiss, Oberkochen, Germany) operating at an acceleration voltage of 2kV. All samples characterized by SEM were coated with 5.5 nm carbon film via physical vapor deposition to avoid the charging effects of the glass. In all cases, the comparison between assemblies were examined at the same laser power (14 mW) and time growth (10 s).

**Surface-Enhanced Raman Spectroscopy:** For SERS measurements, substrates were immersed for 1 h with a drop of aqueous solution of CV (1 µm) and then dried under compressed air. For Raman, a 15 µL drop of an aqueous solution of CV (0.1 m) was pipetted onto a glass substrate, and another glass cover slide was pressed on top of it. SERS and Raman spectra were measured on an inverted microscope with a 60x air objective (NA = 0.85, CFI Plan Fluor, Nikon) using a 785 nm CW laser (LM-785-PLR-175, Ondax, Monrovia, CA) delivering 0.562 mW to the sample, respectively. The scattered light was filtered with either a 532 nm notch filter (NF533-17, Thorlabs, Newton, NJ) and passed through a 10 µm slit. This light was then reflected off of a 650 nm blaze, 1350 lines/mm grating (Richardson
Gratings, Newport, Rochester, NY), in a spectrograph (Kymera 328i, Andor Technology, Belfast, U.K.) and detected using a back-illuminated CCD with a sensor temperature of -80 °C (Newton 970, Andor). Raman signal in the range of 1106–1943 cm⁻¹ was collected and integrated over 3 and 30 s for SERS and Raman, respectively. For SERS measurements, six spectra were obtained for each sample, and the background signal was subtracted for each individual measurement prior to calculation of average and standard deviation. The enhancement factors (EFs) were obtained using the method described in Van Duyne et al.,[32] where EF is defined as (I<sub>SERS</sub>/I<sub>Raman</sub>)·(C<sub>Raman</sub>/C<sub>SERS</sub>). I<sub>Raman</sub> is the Raman Intensity of 0.1 m CV (C<sub>Raman</sub>) on bare glass at 1620 cm⁻¹ and I<sub>SERS</sub> is the Raman Intensity of 1 µm CV (C<sub>SERS</sub>) on the AuNR assemblies.

**Image Analysis of Assembly Growth:** To acquire the radial growth rate of the assemblies, video of the growing assembly was recorded and processed in ImageJ,[33] where the diameter of the growing pattern was measured every 0.5 s, for an average of ten points per sample in the first 5 s of growth. Three different patterns were chosen in every sample.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) (Project ID 447787198), and by the Cluster of Excellence “Advanced Imaging of Matter” of the DFG – EXC 2056 – project ID 390715994. The authors thank Dr. Charlotte Ruhmlieb and Robert Schön of the electron microscopy facility of the University of Hamburg for assistance with SEM measurements.

Open access funding enabled and organized by Projekt DEAL.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

colloids, nanoparticle assembly, plasmonics, SERS, thermophoresis

Received: February 3, 2025

Revised: March 14, 2025

Published online:

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