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. 2026 Aug 24. Online ahead of print. doi: 10.1039/d6na00497k

pH-controlled reversible assembly of magnetic and plasmonic nanoparticles on gold nanorods

Mehedi H Rizvi a, Ruosong Wang b, Jonas Schubert b, Amy L Josefsberg c, Andreas Fery b,d, Joseph B Tracy a,✉
PMCID: PMC13629478  PMID: 42825156

Abstract

Overcoating gold nanorods (GNRs) with plasmonic or magnetic satellite nanoparticles (NPs) can modify the longitudinal and transverse localized surface plasmon resonance (LSPR and TSPR) through coupling with the satellite NPs. We report the use of electrostatic interactions to reversibly assemble different types and amounts of satellite NPs on GNR cores, which allows coupling with and manipulation of the LSPR and TSPR of the GNR core. Positively charged Fe3O4 NPs and spherical gold NPs (GNPs) functionalized with polyethylenimine (PEI) were assembled on the surface of negatively charged GNRs functionalized with bovine serum albumin (BSA). The distinct optical extinction spectra of Fe3O4 NPs and GNRs make straightforward quantification of the loading of satellite NPs possible. Successive overcoatings on GNRs with GNPs followed by Fe3O4 NPs are also demonstrated. pH is a useful parameter for controlling the assembly and loading of satellite NPs because the surface potentials on PEI- and BSA-functionalized NPs strongly depend on pH. The highest loading of satellite NPs is obtained at a pH of 10.8, which is near the isoelectric point (pHI) of PEI (pHI ∼11). Adjusting the pH to much higher or lower values below the pHI of BSA (pHI ∼5) drives disassembly. As a demonstration of the effect of disassembly, diattenuation of transmitted light is observed when GNRs coated with Fe3O4 NPs align in a magnetic field but vanish upon the removal of Fe3O4 NPs, randomizing the orientations of the GNRs.

pH modulates the assembly and disassembly of magnetic and plasmonic nanoparticles on gold nanorods, enabling the dynamic tuning of their properties.graphic file with name d6na00497k-ga.webp

Introduction

There have been major advances in the synthesis and assembly of multifunctional or hybrid nanoparticles (NPs), yet many opportunities remain for tailoring their functional properties and dynamic behaviors.1–5 Multifunctionality includes combining multiple independent functions within a single structure or having coupled properties from which new behaviors can emerge. In core/satellite NPs (CSNPs), several satellite NPs surround a single core NP, which is often larger than the satellites. There are several physical and chemical methods for synthesizing or assembling CSNPs, which also have different extents of reversibility. As an example of a multifunctional CSNP, combining superparamagnetic iron oxide NPs and plasmonic NPs yields magnetically responsive plasmonic NPs that have potential applications in drug delivery and theranostics, combining multimodal imaging and photothermal therapy.6–12 An important emerging application of magnetic–plasmonic hybrid NPs is magnetically controlled chiral assemblies that exhibit dynamic chiroptical properties.13,14 As an example of coupled properties, plasmon coupling in assemblies of plasmonic NPs can alter photon absorption and has emerging applications in biosensing, photovoltaics and lasing.15,16

Common strategies for preparing CSNPs include seeded growth of satellite NPs,9,17,18 heteroaggregation,19 chemisorption,20 and the use of DNA,21–25 peptides,26,27 proteins,28 or other crosslinking molecules.29–36 In seeded growth, heterogenous nucleation and growth of satellite NPs on a core NP are sensitive to the composition and crystal structure of the core.9,37,38 Seeded growth can be challenging because many methods for synthesizing NPs have been optimized for homogeneous nucleation and growth.39,40 The use of crosslinker molecules is subject to constraints imposed by surface chemistry on the core and satellite NPs and may require the fine optimization of several parameters, including the properties of the solvent, to inhibit undesired aggregation.31 The electrostatic assembly of oppositely charged cores and satellite NPs is another approach that has been investigated extensively.41–52 Electrostatic assembly is scalable and does not require maintaining strict reaction conditions. Here, we report the pH-modulated assembly of CSNPs with tunable loadings of cationic magnetic and plasmonic satellite NPs functionalized with polyethylenimine (PEI)53–55 and anionic gold nanorods (GNRs) coated with bovine serum albumin (BSA).56,57

PEI and BSA were chosen for functionalization based on our recent report using them for assembly.57 The positive ζ-potential of the CSNP products keeps them well dispersed in water. PEI is a synthetic polyelectrolyte, and BSA is a protein that is folded under physiological conditions with patches of positive and negative charges that transport several types of molecules in blood, including metal ions and fatty acids. In this work, the behavior of BSA in guiding the assembly of CSNPs is largely consistent with that of an anionic polyelectrolyte, where the effects of its complex chemistry and dynamic structure that changes with the pH and binding to the surfaces of NPs are neglected. It is interesting to note that proteins can retain active, folded structures when bound to the surfaces of NPs.58

The use of external triggers to alter the chemical properties and interactions of NPs allows the dynamic tuning of their multifunctional properties. Dynamic assembly has been reported for single-component NPs triggered by light,59 temperature,60 solvent polarity,61 and pH,62–65 as well as for multicomponent CSNPs triggered by light66 and temperature.67 Tuning the pH allows precise control of the surface charge and has been widely investigated for the assembly of NPs. There have been several reports of reversible clustering of DNA-functionalized NPs by modulating the pH to control hybridization.68–71 While DNA has excellent specificity, it is easily degraded and expensive to scale to large amounts. The pH-controlled agglomeration and disassembly of mixtures of NPs coated with proteins and polyelectrolytes have also been investigated,62–65 on which this work is based. Building on these studies, we report adjusting the pH to regimes where the core and satellite NPs have like charges, driving disassembly and rendering the plasmonic properties insensitive to magnetic fields. This work highlights the versatility of BSA and PEI for pH-driven assembly and disassembly.

Results and discussion

The tunable self-assembly of two different types of satellite NPs, Fe3O4 NPs and spherical gold nanoparticles (GNPs), with GNR cores in water was investigated. The CSNP products with Fe3O4 NPs and GNPs as satellite NPs are referred to as MagGNRs and GNP-GNRs, respectively. The pH during the assembly process was adjusted to achieve CSNPs with different relative amounts of satellite NPs. Adjusting the pH above or below certain limits drives dissociation of the CSNPs.

Assembly of MagGNRs

Satellite NPs, 13.4 nm Fe3O4 NPs and 8.1 nm GNPs, were synthesized by the thermal decomposition of metal salts in the presence of oleic acid and oleylamine, respectively. PEI-functionalized NPs were prepared through a ligand exchange procedure (SI, Fig. S1). The nature of the native ligand during NP synthesis is important for determining ligand exchange behaviors, and some common classifications of different types of ligands are helpful for planning ligand exchange reactions.72,73 Amines found in oleylamine and PEI are L-type (neutral electron donor) ligands, which makes exchange between them favorable and fast. Oleic acid is an X-type (anionic) ligand, which is difficult to exchange with cationic PEI. Oleic acid is useful for synthesizing Fe3O4 NPs of the desired size, but it can be difficult to displace using amines.74 Multiple steps are needed to strip away oleic acid, regraft with oleylamine, and then exchange oleylamine with PEI. Crown ethers coordinate with metal ions and form complexes upon mixing with NaF, generating highly active F− anions, which are efficient in displacing oleic acid from the surface of Fe3O4 NPs.75 Functionalization with a cationic polyelectrolyte like PEI imparts positive ζ-potential and good dispersion in water spanning low pH to mildly basic water. We refer to these PEI-functionalized Fe3O4 NPs as PEI-Fe3O4 NPs. CTAB-stabilized GNRs were prepared using a seed-mediated technique. The positively charged CTAB bilayer was replaced with BSA, which imparts a net negative charge. The capability of BSA to transport hydrophobic molecules makes it well suited for removing CTAB.56 Different functional groups in BSA, such as thiols, amines, and carboxylic acids, together with use of a CTAB concentration below the critical micelle concentration, drive the overcoating of GNRs with BSA, which we call BSA-GNRs.56,57 Unless otherwise mentioned, BSA-GNRs with a core size of 76 nm × 26 nm were used to synthesize MagGNRs (SI, Fig. S2a).

Upon mixing negatively charged BSA-GNRs and positively charged PEI-Fe3O4 NPs, the attractive electrostatic interactions between them drive self-assembly into MagGNRs. Excess PEI-Fe3O4 NPs were then purified by centrifugation several times, discarding the supernatant and dispersing the pellet in deionized water. Excess PEI-Fe3O4 NPs are required to achieve fully overcoated MagGNRs with minimal bridging or agglomeration (SI, Fig. S3). The extinction spectra of purified MagGNRs resemble BSA-GNRs with two distinctions (Fig. 1a). Fe3O4 NPs contribute prominently to the spectra of MagGNRs in the blue region at wavelengths below 600 nm. The longitudinal surface plasmon resonance (LSPR) of MagGNRs is red-shifted because PEI-Fe3O4 NPs increase the dielectric constant on the surface of the GNR.30,76 Furthermore, the loading of PEI-Fe3O4 NPs in MagGNRs depends on the pH during assembly. With the increasing loading of PEI-Fe3O4 NPs, the LSPR of MagGNRs is red-shifted (Fig. 1a and b). Previous studies have shown that dielectric shells around GNRs have little effect on the extinction of GNRs except for the redshift.77,78

Fig. 1. pH-dependent assembly of MagGNRs: (a) extinction spectra of PEI-Fe3O4 NPs, BSA-GNRs and purified MagGNRs (normalized at the LSPR maximum). (b) Number of PEI-Fe3O4 NPs attached to each BSA-GNR and shift of LSPR and TSPR in MagGNRs. (c) ζ-Potential of PEI-Fe3O4 NPs and BSA-GNRs as a function of pH. (d–g) TEM images of MagGNRs assembled at the pH values of 9.32, 10.07, 10.81, and 12.11.

Fig. 1

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The distinct extinction spectra of Fe3O4 NPs and the LSPR of GNRs (Fig. 1a) are the basis for developing a technique to estimate the loading of Fe3O4 NPs in MagGNRs at different pH values (Fig. 1b). We can approximate the extinction spectrum of MagGNRs as a linear combination of the spectra for BSA-GNRs and PEI-Fe3O4 NPs, AMagGNRs(λ) ≈ αABSA-GNRs(λ) + βAPEI-Fe3O4 NPs(λ), where A, λ, α, and β are the extinction, wavelength, and the relative contribution of BSA-GNRs and PEI-Fe3O4 NPs in the spectra of MagGNRs, respectively. With α and β from the above equation and extinction coefficients for BSA-GNRs and PEI-Fe3O4 NPs, one can estimate the relative amount of the constituent NPs in MagGNRs (SI, Fig. S4). This approach is reasonable, because we have previously shown that Fe3O4 NPs do not substantially alter the LSPR except for the redshift discussed above.57

The loading of PEI-Fe3O4 NPs increases with increasing pH up to 10.81 and then decreases, which could be explained by the pH dependence of the ζ-potential of the core and satellite NPs (Fig. 1b and c). The isoelectric point (pHI) of BSA is 4.7,79,80 which agrees with our measurements of the ζ-potential of BSA-GNRs at different pH values. As the pH increases, the negative ζ-potential of the deprotonated BSA-GNRs reaches a constant value of ∼−26 mV. The positive ζ-potential of protonated PEI-Fe3O4 NPs decreases with increasing pH, crossing zero near a pHI of ∼10.9, followed by a further decrease. In the pH range of 9–11, the core and satellite NPs undergo spontaneous self-assembly upon mixing because they are oppositely charged. The increased loading of PEI-Fe3O4 NPs in MagGNRs as the pH increases to 10.81 is caused by the decreasing surface charge on the PEI-Fe3O4 NPs, thus requiring more PEI-Fe3O4 NPs to neutralize the negative charge of the BSA-GNRs. Reduced repulsion between the PEI-Fe3O4 NPs also allows higher loading as their ζ-potential decreases.

Especially near neutral ζ-potential, other attractive interactions can become important, including van der Waals interactions, interactions between the BSA and PEI polymer chains, and magnetic interactions between PEI-Fe3O4 NPs.81 These other interactions can dominate over repulsive interactions of weak, like charges. When attempting assembly at pH 12.11, some PEI-Fe3O4 NPs bind to BSA-GNRs, despite both having negative charges. At this pH, the loading of PEI-Fe3O4 NPs is much lower, however, than that at 10.81.

The maximum loading is achieved near the pHI of PEI-Fe3O4 NPs, where they still have a small, positive surface charge. The LSPR shift saturates at pH 10.07, before the loading of PEI-Fe3O4 NPs is maximized, which suggests the formation of multilayers in this saturated regime. A maximum red-shift of ∼24 nm was observed when BSA-GNRs were entirely covered with at least a single layer of PEI-Fe3O4 NPs. There is a much smaller shift in the TSPR (≤2 nm) that follows the same trend, but for the LSPR, the shift is much larger, and there is less possibility of interference with the extinction from PEI-Fe3O4 NPs in the blue region.

The TEM images of MagGNRs confirm that the loading of PEI-Fe3O4 NPs increases with the pH and then decreases (Fig. 1d–g). At pH 9.32, the CSNPs are not fully covered with a layer of PEI-Fe3O4 NPs, and approximately a monolayer is obtained at pH 10.07, though TEM does not provide a solvated structure in three dimensions. For assembly at pH 10.81, the coating consists of a multilayer of PEI-Fe3O4 NPs, but assembly at pH 12.11 gives a minimal loading of satellite PEI-Fe3O4 NPs.

Disassembly of MagGNRs

MagGNRs are stable in deionized water for at least several months, where both core and satellite NPs are oppositely charged. However, the dissociation of CSNPs can be triggered by decreasing (increasing) the pH of the environment, where both core and satellite NPs are fully protonated (deprotonated) with high positive (negative) surface charges. Repulsive core–satellite and satellite–satellite interactions drive fast dissociation of MagGNRs into BSA-GNRs and PEI-Fe3O4 NPs (Fig. 2). Then, 1 M HCl (1 M NaOH) was added into the MagGNR dispersion with a micropipette to lower (raise) the pH to 2.1 (12.4), which drives immediate dissociation. The extinction spectra show a corresponding blue-shift (Fig. 2a and b). The dissociated product was purified by centrifuging and then discarding the supernatant containing unbound PEI-Fe3O4 NPs, followed by redispersion in deionized water. The extinction spectra of the purified dissociated samples closely resemble BSA-GNRs with a slight redshift in the LSPR of ≤3 nm corresponding to a small portion of undissociated PEI-Fe3O4 NPs. The TEM images of the purified samples also confirm nearly complete dissociation of MagGNRs (Fig. 2c and d). A small number of PEI-Fe3O4 NPs remain despite disassembly and might be trapped, e.g., through entanglement of the BSA and PEI coatings.

Fig. 2. pH-triggered disassembly of MagGNRs: (a and b) extinction spectra of BSA-GNRs and MagGNRs (normalized at the LSPR maximum) before adjusting the pH to 2.1 or 12.4 and disassembled MagGNRs after adjusting the pH, with the spectra acquired before and after purification. (c and d) TEM images of MagGNRs disassembled at pH of 2.1 and 12.4 after purification.

Fig. 2

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Successive cycles of assembly and disassembly might be possible, but no such experiments are reported, because significant challenges are anticipated. Assembly is conducted with an excess of satellite NPs, but there is no excess present in the products of disassembly. Furthermore, adjusting the pH involves the addition of acids or bases, which increases the ionic strength of the solution. This would shield the charges, thus altering and likely inhibiting electrostatic assembly.

MagGNRs with larger cores can align in magnetic fields, which makes control of their anisotropic optical properties with magnetic fields possible. The optical extinction of large MagGNRs strongly depends on the angle between the directions of the magnetic field and light polarization. Specifically, MagGNRs preferentially absorb light at LSPR wavelengths polarized parallel to the magnetic field and TSPR wavelengths polarized perpendicular to the magnetic field.57 This magnetically induced optical anisotropy of MagGNR solutions can be observed in the light transmitted between crossed polarizers. MagGNRs were prepared with BSA-GNR core dimensions of 153 nm × 46 nm (SI, Fig. S2b). A cuvette with aqueous MagGNRs is placed between crossed polarizers within the gap of an electromagnet, depicted schematically in Fig. 3a. The magnetic field points in the plane of polarizers at an angle of 45° with respect to the transmission axis of each polarizer. Light from an array of small LEDs successively passes through a polarizer, a cuvette, and a second, crossed polarizer, and finally enters the camera. The following sequence is shown in SI, Movie S1. When the electromagnet is off, MagGNRs are randomly oriented, and the crossed polarizers block most of the light coming from the LEDs to the camera (Fig. 3b). Switching the electromagnet on orients MagGNRs at 45° with respect to both polarization directions (Fig. 3a) and acts to preferentially absorb (transmit) horizontally polarized light and transmit (absorb) vertically polarized light at LSPR (TSPR) wavelengths, thus acting as an intervening vertical (horizontal) polarizer. By the 3-polarizer paradox,82 both LSPR and TSPR wavelengths consequently exhibit higher transmission than in the absence of the magnetic field, and the LEDs appear bright (Fig. 3c). Upon adding a drop of 1 M HCl while keeping the electromagnet on (Fig. 3d), the color begins to fade, because the acid drives the dissociation of MagGNRs, making them unable to align with the magnetic field and eliminating diattenuation. Accelerating mixing with a pipette completes dissociation, and the transmitted light from each LED returns to its initial dim state (Fig. 3e). Switching off the electromagnet has no further effect on the dissociated MagGNRs (Fig. 3f).

Fig. 3. (a) Schematic of the setup for diattenuation experiments. (b–f) Photographs of a cuvette with 1 cm path length containing aqueous large MagGNRs placed between crossed polarizers (polarization directions indicated by diagonal lines) and in the gap of the electromagnet in front of an array of three discrete white LEDs: (b) magnetic field off, (c) field on (2 kOe), (d) field on immediately after adding a drop of 1 M HCl to drive the dissociation of MagGNRs, (e) after mixing and (f) after turning the field off.

Fig. 3

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Assembly and disassembly of GNP-GNRs

As a test of the generality of electrostatic assembly of PEI-functionalized satellite NPs on BSA-GNRs, PEI-GNPs were assembled with BSA-GNRs, yielding GNP-GNRs. In both MagGNRs and GNP-GNRs, the pH determines the ζ-potential and loading of satellite NPs, and can control disassembly. The loading of PEI-GNPs on GNP-GNRs initially increases with the pH during assembly and then gradually decreases (Fig. 4a). The maximum loading of PEI-GNPs is obtained at pH 10.78, which is close to the pHI of PEI-GNPs (Fig. 4b). TEM images of the GNP-GNRs confirm pH-dependent loading of PEI-GNPs, in agreement with the extinction spectra (Fig. 4c–g). Near-field coupling, both core–satellite and satellite–satellite interactions, yields a red-shift in the LSPR and TSPR of GNP-GNRs.28 Experiments and simulations on related GNP-GNR structures also result in redshifts in the LSPR and TSPR.49,83 Unlike MagGNRs, in GNP-GNRs, the extent of coupling and magnitude of the redshift do not saturate as the loading of PEI-GNPs increases (Fig. 4a and b). The distinct behaviors for the redshifts in MagGNRs and GNP-GNRs are not surprising given the different underlying mechanisms. The disassembly behavior of GNP-GNRs is the same as that of MagGNRs. At pH 2.12 and 12.38, the GNP-GNRs are nearly completely dissociated (SI, Fig. S5).

Fig. 4. pH-dependent assembly of GNP-GNRs: (a) extinction spectra of PEI-GNPs, BSA-GNRs, and purified GNP-GNRs (normalized at the LSPR maximum). (b) Red shift of LSPR and TSPR maximum overlapped with the ζ-potential of BSA-GNRs and PEI-GNPs at different pH values. (c) Magnified TEM image of GNP-GNRs assembled at pH 10.78. (d–g) TEM images of GNP-GNRs assembled at the pH values of 9.21, 10.33, 10.78, and 12.09.

Fig. 4

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Successive overcoatings of GNPs and Fe3O4 NPs

Combining multiple types of satellite NPs in CSNPs can further expand or tailor the multifunctional properties. As an example of such assembly, BSA-GNRs are sequentially coated with PEI-GNPs and then PEI-Fe3O4 NPs. After assembling GNP-GNRs at pH 10.61 and purification, they were mixed with PEI-Fe3O4 NPs and the pH was adjusted to 10.98 to deposit an additional layer of PEI-Fe3O4 NPs onto the GNP-GNRs. The composite Fe3O4-GNP-GNRs were then centrifuged to remove excess PEI-Fe3O4 NPs, followed by redispersion in deionized water. The extinction spectrum of Fe3O4-GNP-GNRs shows a shoulder in the blue region from the loading of PEI-Fe3O4 NPs (Fig. 5a). In addition to the 48 nm redshift in the LSPR from the PEI-GNP coating, the PEI-Fe3O4 NPs cause a further 10 nm redshift, which is attributed to the altered dielectric environment surrounding the GNP-GNRs. The TSPR also exhibits an additional 4 nm redshift due to the additional layer of PEI-Fe3O4 NPs. The TEM images confirm that PEI-GNPs remain bound to BSA-GNRs after the deposition of PEI-Fe3O4 NPs, and the satellite NPs do not mix within the coating (Fig. 5c).

Fig. 5. (a) Extinction spectra of BSA-GNRs, GNP-GNRs, and Fe3O4-GNP-GNRs (normalized at the LSPR maximum). (b) TEM image of GNP-GNRs before adding PEI-Fe3O4 NPs. (c) TEM image of composite Fe3O4-GNP-GNRs with an inner layer of GNPs and an outer layer of PEI-Fe3O4 NPs.

Fig. 5

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Multilayers of satellite NPs

The above-mentioned results (e.g.Fig. 1f, 4c, e, f and 5c) appear to show multilayers of satellite NPs. These images show the structures after drying onto flat TEM substrates, however, and the positions of the satellite NPs could shift during drying. Cryo-TEM further confirms the presence of a multilayer of satellite NPs (SI, Fig. S6). Chain-like structures of PEI-Fe3O4 NPs in these images suggest that magnetic interactions could play a role in the formation of these structures. Electrostatic assembly would appear to prohibit the formation of multilayers of like-charged NPs, because the first layer would be expected to repel additional satellite NPs. As already discussed, other interactions may dominate over electrostatic interactions. It is also important to note that long-range electrostatic interactions might also promote the deposition of multilayers of satellite NPs under certain conditions. In related systems, the electrostatic potential surrounding the charged core NP penetrates well beyond the surface, several tens to hundreds of nm, and can trap oppositely charged satellite NPs from large separation distances.84–87 The depth of the potential well increases with the increasing surface charge of the core NPs, corresponding to a larger ζ-potential.84,85 While there is a repulsive electrostatic potential between like-charged satellite NPs that would resist the deposition of additional satellite NPs, the region over which this potential is strong enough to affect the assembly behavior becomes thinner as the magnitudes of the surface charges of the core and satellite NPs increase and decrease, respectively.87 Therefore, especially in the regime where the PEI-Fe3O4-NPs have a small, positive ζ-potential, the negatively charged BSA-GNR core may dominate the electrostatics and support the formation of multilayers of satellite NPs.

Conclusions

Self-assembly based on electrostatic interactions is a powerful method for synthesizing hybrid materials with emergent properties that cannot be obtained from individual components. By applying coatings to inorganic NPs bearing accessible acid/base groups, pH control over the surface potential and thus over self-assembly becomes possible and is reversible. Through controlling the surface potential, this work addresses a challenge in the controlled loading of satellite NPs in CSNPs. Functionalization of GNRs with BSA imparts a net negative charge, and functionalization of Fe3O4 NPs with positively charged PEI allows controlled assembly over a wide pH window, in which the components are oppositely charged. Outside of this pH window, both types of NPs have the same charge, which inhibits the assembly and can drive the disassembly of CSNPs. Although BSA is a complex protein, its behavior in this work is consistent with an anionic polyelectrolyte. Therefore, this approach could likely be applied for developing other types of CSNPs with tunable multifunctional properties. These pH-sensitive CSNPs could be integrated with other pH-responsive systems or used in sensing, drug delivery, or photothermal therapy applications.

As an example emphasizing the multifunctional responsiveness of MagGNRs to both magnetic fields and pH, MagGNRs could potentially be transported over long distances with externally applied magnetic field gradients, followed by pH-triggered disassembly. The GNRs could be delivered to a specific location for photothermal heating, or a molecular payload could be released. If MagGNRs would be transported into an environment with a different pH, automated disassembly could be envisioned. Such a scheme would likely involve altering the coatings on the core and satellite NPs to engineer pH responsiveness that would match the change in the pH of the environment.

Experimental section

Chemicals

Cetyltrimethylammonium bromide (CTAB, Amresco, high purity), HAuCl4·xH2O (Alfa Aesar, 99.999%, where x was estimated as 3), KBr (Alfa Aesar, ACS, 99% min), AgNO3 (Alfa Aesar, 99.9995%), deionized water (Ricca, ACS Reagent grade, ASTM Type I, ASTM Type II), ascorbic acid (AA, J.T. Baker, 99.5%), hydroquinone (Acros Organics, 99.5%), NaBH4 (Sigma-Aldrich, 99%, 213 462), NaOH (Sigma-Aldrich, 98%), BSA (Sigma-Aldrich, A7906, 98%), Na citrate dihydrate (Mallinckrodt, 99% min), FeCl3 (Alfa Aesar, anhydrous, 98%), Na oleate (TCI America, 97% min), oleic acid (Sigma-Aldrich, 90%), 1-octadecene (Sigma-Aldrich, 90%), benzyl ether (Acros Organics, 99%), oleylamine (Sigma-Aldrich), hexanes (Macron, 95%), ethanol (Koptec, absolute, anhydrous), isopropanol (Macron, 99.5%), chloroform (Sigma-Aldrich, OmniSolv, 99.9%), 15-crown-5 ether (Ambeed, 98%), NaF (Alfa Aesar, 99.99%), and PEI (Alfa Aesar, branched, MW 10000, 99%) were used as received. Note: In the following experiments, deionized water was used wherever water is mentioned.

Synthesis of CTAB-stabilized GNRs

CTAB-GNRs with a LSPR of 736 nm and dimensions of 74 nm × 26 nm were synthesized in a three-step growth process. In the first step, 19 mg of CTAB-GNRs was synthesized following an established method with an LSPR of 760 nm by prolonging the duration of injection of ascorbic acid during secondary growth to 260 min.88 Then, 20 mL of the unpurified CTAB-GNRs were mixed with 10 mL of tertiary growth solution containing 91 mM CTAB, 0.93 mM HAuCl4, 9.1 mM KBr, and 0.992 mM ascorbic acid and aged for 2 hours to achieve the final dimensions and LSPR wavelength. The GNR solution has a CTAB concentration of 91 mM and required purification by centrifugation to achieve a CTAB concentration of ∼1 mM and a final volume of 10 mL for further overcoating with BSA. Larger BSA-GNRs with dimensions of 153 nm × 46 nm (used only in Fig. 3 and SI, Movie S1) were prepared and functionalized with BSA following a recently published overgrowth procedure.57

BSA overcoating of GNRs

The procedure for BSA overcoating of GNRs is based on a reported method,56 which we have recently modified for assembling MagGNRs.57 First, 20 mg of BSA was dissolved in 20 mL of water containing 0.02 wt% Na citrate under sonication. Next, 10 mL of water was added to the previously prepared 10 mL of CTAB-GNRs with its CTAB concentration of ∼1 mM, which was then quickly added to the BSA solution under sonication. The mixture was sonicated for 30 min, and ice was added to the sonication bath intermittently to keep the temperature below 35 °C. The GNR solution was centrifuged (Thermo Scientific Sorvall Legend X1R with Fiberlite F15-6x100y rotor) at 9000 rpm (8875g) for 20 min to remove the supernatant. A volume of 20 mL of a 0.1 mg per mL BSA solution was prepared with its pH adjusted to ∼10 by adding NaOH, into which the GNR pellet was redispersed and aged overnight. This solution was purified four times to remove excess BSA using the same centrifuge speed, duration of 20 min, and redispersion in 20 mL of pH (∼10)-adjusted water, after which the redispersed BSA-GNRs were stored in a refrigerator.

Synthesis of Fe3O4 NPs and functionalization with PEI

Oleic acid-stabilized 13.4 nm Fe3O4 NPs were synthesized by thermal decomposition of Fe oleate in an organic solvent with a high boiling point.89,90 Fe oleate was prepared by dissolving 18.27 g of Na oleate and 3.244 g of anhydrous FeCl3 in 40 mL of ethanol, 70 mL of hexanes, and 30 mL of water. The biphasic mixture was refluxed at 70 °C for 4 hours with magnetic stirring to complete the reaction. The organic layer containing Fe oleate was then washed three times with water to remove unreacted Na oleate and dried in a fume hood for two days to remove hexanes. For synthesizing Fe3O4 NPs, 2.5 g of this Fe oleate and 1.26 g of oleic acid were dissolved in 5.55 mL of 1-octadecene and 2.77 mL of benzyl ether in a round-bottomed flask connected to a Schlenk line. The mixture of reactants was degassed under vacuum at 60 °C for 30 min, followed by backfilling with N2 and heating at a rate of 3.3 °C min−1 to 325 °C for 30 min. After cooling to room temperature, 2 mL of the product was added to premixed hexanes (2 mL) and ethanol (6 mL) and briefly centrifuged to isolate the NPs from unreacted precursors that form a dark supernatant, followed by redispersion in 20 mL hexanes.

The oleic acid ligands on Fe3O4 NPs were stripped away and regrafted with oleylamine to facilitate PEI functionalization. First, 14 mL of isopropyl alcohol was added to 20 mL of Fe3O4 NPs in hexanes, followed by the addition of a solution containing 2 mL of water, 100 mg of NaF, and 0.45 mL of 15-crown-5 ether. The mixture was sonicated for 5 min and then aged for 2 hours without mixing to complete ligand stripping, after which the agglomerated NPs were successively washed with hexanes, isopropanol, water, and ethanol. Washing was performed twice in each solvent, where the NPs were sonicated for 10 min, centrifuged at 5000–8000 rpm (2739–7012g), and then redispersed with brief sonication in the next solvent after discarding the supernatant. After the washing step with ethanol, 9.98 mL of chloroform was added to the sedimented Fe3O4 NPs, followed by sonication and the addition of 20 µL of oleylamine. The NPs were quickly regrafted with oleylamine, which turned the turbid suspension into a brown-red transparent solution of dispersed NPs.

To complete functionalization with PEI, 100 mg of PEI was dissolved in 30 mL of chloroform for functionalizing Fe3O4 NPs. An amount of 9 mL of chloroform was added to 1 mL of the stock solution of oleylamine-stabilized Fe3O4 NPs, followed by mixing with the whole PEI solution under vigorous stirring. The mixture was then sonicated for 2 hours while maintaining the temperature below 40 °C, followed by stirring for 2 hours at 40 °C. After cooling to room temperature, 5 mL of water was added, followed by shaking by hand for 2 min. PEI-functionalized Fe3O4 NPs separated into the aqueous phase, which was extracted and washed 5 times to remove excess PEI. In each washing cycle, centrifugation was performed using a centrifuge membrane filter (Amicon Ultra, 100 kDa) at 3500 rpm (1342g) for 30 min. Excess PEI molecules passed through the filter, while the PEI-Fe3O4 NPs were collected on the filter. Between washing cycles, 5 mL of water was added to redisperse the PEI-Fe3O4 NPs. The purified NPs were redispersed in 2.5 mL of water.

Synthesis and PEI functionalization of GNPs

Oleylamine-stabilized GNPs with an average diameter of 8.1 nm were synthesized by a hot-injection method.91 First, 4 mL of oleylamine was degassed at room temperature for 30 min and then heated to 150 °C under N2. A solution of 2 mL of oleylamine containing 150 mg of HAuCl4·xH2O was injected in the hot stirring oleylamine and allowed to react for 7 min. After rapid cooling to room temperature, 5 mL of hexanes and then 15 mL of ethanol were added to flocculate the GNPs, which were isolated by brief, low-speed centrifugation. The product was redispersed in 5 mL of chloroform.

PEI functionalization was performed in a similar fashion to the oleylamine-stabilized Fe3O4 NPs. First, 0.2 mL of the GNP stock solution was diluted in 10 mL of chloroform and added to 30 mL of chloroform containing 25 mg of PEI under vigorous stirring. The mixture was sonicated for 30 min while maintaining the temperature below 40 °C and stirred for 2 hours at 40 °C. The PEI-GNPs were purified using centrifuge membrane filters and redispersed in 2 mL of water.

Assembly and disassembly of CSNPs

A series of BSA-GNRs dispersed in water with different basic pH were prepared by taking 0.5 mL of the purified BSA-GNRs, centrifuging (Eppendorf 5415C) them at 5000 rpm (2152g) for 10 min and replacing the supernatant with 0.5 mL of basic water containing 0.01–20 mM of NaOH. Each sample in the series of BSA-GNR solutions was mixed with 0.5 mL of PEI-Fe3O4 NPs or PEI-GNPs in water, and the pH was registered after mixing but before purification. After stirring each mixture of core and satellite NPs for 2 hours, 4 cycles of centrifugation at 5000 rpm (2152g) for 10 min with dispersion in 1 mL of water were performed to separate CSNPs from excess PEI-Fe3O4 NPs or PEI-GNPs. After the final purification cycle, the product was redispersed in 2 mL of water for further characterization. Disassembly occurs quickly when the pH of purified CSNPs is increased (decreased) by adding 1 M NaOH (1 M HCl) dropwise. The mixture was centrifuged at 5000 rpm (2152g) for 10 min to remove the supernatant containing disassembled satellite NPs. The sedimented GNRs were then redispersed in 2 mL water.

Quantification of the loading of satellite NPs

The values of α and β were chosen to achieve maximum overlap between the sum of the extinctions, αABSA-GNRs(λ) + βAPEI-Fe3O4 NPs(λ), and AMagGNRs(λ). The concentrations of core and satellite NPs can be calculated from α, β, and the extinction coefficients of BSA-GNRs and PEI-Fe3O4 NPs, giving the average number of satellite PEI-Fe3O4 NPs in a MagGNR (SI, Fig. S4). A solution of Fe3O4 NPs with an extinction of 20 (calculated from quantitative dilution) at 400 nm has a concentration of 0.98 mg Fe3O4/mL from inductively coupled plasma optical emission spectroscopy (ICP-OES). Assuming that the diameter of the spherical Fe3O4 NPs is 13.4 nm and using the bulk density of 5.17 g cm−3, the concentration of Fe3O4 NPs corresponding to each extinction value can be estimated. Similarly, for GNRs, an extinction of 1.2 at 400 nm corresponds to a concentration of 0.0985 mg mL−1, which is 0.50 mM of Au atoms.92

Assembly of Fe3O4-GNP-GNRs

First, 0.5 mL of purified GNP-GNRs dispersed in water was added to a stirring solution of 0.5 mL PEI-Fe3O4 NPs. A few microliters of 0.05 M NaOH was incrementally added to the stirring mixture, until a pH of 11 was achieved. After stirring for 2 hours, the excess PEI-Fe3O4 NPs were removed by 4 cycles of centrifugation at 3500 rpm (1342g) for 20 min. After discarding the supernatant, the Fe3O4-GNP-GNRs were redispersed in 2 mL of water.

Characterization

Transmission electron microscopy (TEM) was performed using an FEI Talos F200X G2 at an operating voltage of 200 kV. The samples were prepared by drop-casting followed by drying onto carbon-coated Cu TEM grids. Cryo-TEM images were acquired using a Carl Zeiss Libra 120 microscope at an accelerating voltage of 120 kV. The samples were prepared on carbon-coated holey carbon TEM grids (Quantifoil R2/2) by shock freezing in liquid ethane in a Leica GP plunge freezer and imaged using a Gatan 626 cryo-TEM holder at −170 °C in the energy-filtered (zero-loss) bright field. Optical extinction spectroscopy was performed using an Ocean Optics Red Tide USB650 spectrometer with a glass cuvette having a 1 cm path length. The ζ-potential was measured using a Malvern Zetasizer Nano. For the experiment showing dichroism (Fig. 3 and SI, Movie S1), large MagGNRs were prepared by mixing 0.75 mL of PEI-Fe3O4 NP aqueous stock solution and 0.75 mL of large BSA-GNRs (prepared as described previously)57 dispersed in basic water. The mixture was stirred for 30 min, and a pH of 10.6 was recorded. The solution was then transferred into a cuvette and diluted with 1.5 mL of water. The cuvette was sandwiched between crossed polarizers (Edmund Optics 43-781) and placed between the poles of a GMW 3472-70 electromagnet.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

NA-OLF-D6NA00497K-s001
Download video file (5.8MB, mov)
NA-OLF-D6NA00497K-s002

Acknowledgments

We gratefully acknowledge Petr Formanek for cryo-TEM measurements, Kim Hutchison for ICP-OES measurements, and Christian Rossner for helpful discussions. This research was supported by the National Science Foundation (CMMI-1763025), Alexander von Humboldt Foundation, Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) GRK 2767 – Projektnummer 451785257 and EXC 3035 Responsible Electronics in the Climate Change Era (REC2) – Projektnummer 533607596, Leibniz Research Alliance Advanced Materials Safety, and China Scholarship Council (201808080193). This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (ECCS-2025064). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). This article is dedicated to the memory of Professor Zijie Yan, with whom we had planned to collaborate.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: additional TEM images and extinction spectra, video of pH-triggered disassembly of MagGNRs between crossed polarizers. See DOI: https://doi.org/10.1039/d6na00497k.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

NA-OLF-D6NA00497K-s001
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NA-OLF-D6NA00497K-s002
NA-OLF-D6NA00497K-s002.pdf (1.3MB, pdf)

Data Availability Statement

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: additional TEM images and extinction spectra, video of pH-triggered disassembly of MagGNRs between crossed polarizers. See DOI: https://doi.org/10.1039/d6na00497k.

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