One-dimensional assembly of polymeric ionic liquid capped gold nanoparticles driven by electrostatic dipole interaction

Abstract

In this paper, a bottom-up method for preparing a one-dimensional plasmonic nanostructure is described using a gold nanoparticle capping agent poly[1-methyl 3-(2-methacryloyloxy propylimidazolium bromine)] (PMMPImB-@-Au NPs) as a building block. The core–shell particles were prepared via a “grafting-to” approach, and the structures were confirmed by dynamic light scattering, UV/vis spectroscopy, transmission electron microscopy and thermogravimetric analysis. The development of new plasmonic coupling absorption in the higher-wavelength region and TEM analysis clearly reveal that the PMMPImB-@-Au NPs form chain-like assemblies after the addition of HPO₄²⁻. The effects of different anion valences and polymer chain lengths, the structure of gold cores that have been removed and the change in the particle surface potentials were investigated to determine the mechanism of assembly. The results indicate that some divalent and polyvalent anions, such as HPO₄²⁻, SO₄²⁻, CO₃²⁻, PO₄³⁻ and P₂O₇⁴⁻, can serve as a molecular bridge for electrostatic coupling with two imidazolium groups from adjacent particles, and electrostatic dipole interaction is the major driving force for the self-assembly of PMMPImB-@-Au NPs with chain-like arrays.

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1. Introduction

Fabricating semiconductor^1–3 or metal nanoparticles^4–6 into ordered arrays or predesigned structures has been a promising strategy for developing next-generation nanotechnologies.^7–9 The plasmonic nanostructures with one-dimensional (1D),^10,11 two-dimensional (2D)^12,13 or three-dimensional (3D)¹⁴ architecture have shown distinct properties, different from a simple collection of individual particles and have achieved the potentials of isolated nanoparticles.¹⁵ In particular, the construction of 1D plasmonic nanostructure has received considerable interest because the plasmonic nanostructures produce an electromagnetic focusing effect caused by the plasmon coupling, which provides promising application in nanoelectronics, optoelectronics, nanomagnetism and sensor devices.^16–19 Moreover, understanding the nanoconjugation mechanisms from sphere to chain offered an important opportunity for mimicking the self-assembly process inside the organism,^20,21 and representing the reaction process of atoms and molecules through the plasmonic properties of the particle.²²

Currently, the bottom-up approach for fabricating a 1D nanostructure includes the selective decoration of a spherical surface feature, followed by linking the nanostructures with a molecular bridge in a condensation reaction²³ or directing them by means of a template^6,24 or an exterior force.²⁵ To date, the surface bonding with thiolated organic molecules²⁶ and single strand oligonucleotides¹² was applied for creating such nanomaterials through reliable interparticle interactions.²⁷ Generally, the sizes of capping material and linker molecule determine the inter-particle distance, and thus they affect the coupling properties and applications.^15,28,29 The use of short capping molecules is expected to decrease the interparticle distance, give rise to strong plasmon coupling and improve collective vectorial properties. In addition, the properties can also be tailored and optimized by the space modifier that is utilized. For example, the interparticle distance can be tuned with the aid of special structures and properties of the polymer to control the assembly of nanoparticles with short capping reagents.^30,31 However, further advances in the controlled fabrication of larger size and functional materials capping nanoparticle-based structures are still needed.

The synthesis of core–shell particles comprising a gold nanoparticle (Au NP) core and a polymer shell has recently gathered great scientific interest with a view towards not only improving the dispersion stability of the nanoparticles in aqueous media, but also towards offering new opportunities for constructing the functional nanomaterials and applications.^32–34 The adjustable chemical composition and function of the polymer shell has multiple options available to fabricate such materials via interactions with the surrounding media or self-organization with other substances. For example, environmentally sensitive self-aggregated systems can be sharply responsive to the change of temperature, pH value, ionic strength and light,^35–38 based on the distance dependent surface plasmon resonance (SPR). The core–shell particles are also designed as ideal drug³⁹ and gene delivery⁴⁰ carriers due to their advantages in tumour-targeting and controllable release.⁴¹ Assembling core–shell particles and producing near-infrared absorption caused by the plasmon coupling of Au cores has revealed important biomedical applications.⁴² For instance, He et al.⁴³ obtained new hierarchical functional materials, including unimolecular micelles, clusters and vesicles, via the self-assembly of amphiphilic block copolymers coating on Au NPs in selective solvents. The materials showed tunable absorption in the near-infrared range by controlling the length of the polymer and the size of the Au NPs; thus, facilitating applications in bio-imaging and photo-thermal therapy for cancer. However, the disordered distribution of Au NPs in assemblies resulted in their plasmonic coupling, occurring only under the special set of assembling parameters. If polymer coated Au NPs were fabricated into 1D plasmonic nanostructures, it would be easy to access the plasmonic coupling property and their subsequent applications in biomedicine. In addition, the size of the polymer can be tuned through synthetic procedures, convenient for tailoring interparticle distance and absorption peaks in the near-infrared range. However, to the best of our knowledge, only few related studies about this 1D plasmonic nanostructure have been reported to date.

Here, we describe a simple method to fabricate chain-like plasmonic nanostructures using polymeric ionic liquids (PILs) monolayer-protected Au NPs (PILs-@-Au NPs) as the building block. PILs are new functional polymers prepared by introducing the ionic liquid species in the repeating units.^44,45 Study of the interaction of PILs and anions have been applied to the design of new anion-sensitive smart materials, including hydrophilic/hydrophobic switchable surfaces, block copolymer micelles, optical sensors and reversible porous polymers.⁴⁴ In the present study, the core–shell particle, comprised of an Au NPs core and a poly[1-methyl-3-(2-methacryloyloxy propylimidazolium bromine)] (PMMPImb-@-Au NPs) shell, was prepared by reducing chloroauric acid with a borane-tert-butylamine (BTBA) complex and then attaching PMMPImb to their surface through the techniques of the “grafting to” approach. The assembling of particles into chain-like arrays can be driven by HPO₄²⁻ as a molecular bridge via an electrostatic dipole interaction with imidazolium cations between adjacent particles. The process of preparing PMMPImb-@-Au NPs and forming the chain-like nanostructure is illustrated in Scheme 1.

Scheme 1 Schematic representation of preparing PMMPImb-@-Au NPs and forming a chain-like nanostructure via electrostatic dipole interaction.

2. Experimental section

Materials

2,2-Azobisisobutyronitrile (AIBN) was purchased from J&K Chemical and recrystallized before use as an initiator. 3-Bromo-1-propanol (95%), 1-methylimidazole (98%), methacryloylchloride (97%), and triethylamine (99.5%), were purchased from the Chemical Reagent Company of Shanghai, China. Borane-tert-butylamine complex and hydrogen tetrachloroaurate (HAuCl₄·3H₂O) were purchased from Sigma-Aldrich and used as received. The metal salts Na₂HPO₄, Na₃PO₄, Na₄P₂O₇, Na₂SO₄, Na₂CO₃, NaH₂PO₄, NaHCO₃, NaHSO₄, NaBF₄, NaNO₃ and NaPF₆ were purchased from National Pharmaceutical Group Chemical Reagents. DMF, THF, and DMSO were purchased from the Chemical Reagent Company of Shanghai, China, and used as received. All glassware were cleaned with freshly prepared aqua regia and thoroughly rinsed with Milli-Q water prior to use. The ionic liquid monomers, 1-methyl-3-(2-methacryloyloxy propylimidazolium bromine) (MMPImB), and the chain transfer agent used, (4-cyanopentanoic acid)-4-dithiobenzoate (CPAD), were synthesized according to the procedures mentioned in the literature.^46,47

Synthesis of poly[1-methyl 3-(2-methacryloyloxy propylimidazolium bromine)]

The chain transfer agent CPAD (0.11 g, 0.04 mmol), AIBN (1.6 mg, 0.009 mmol), MMPImB (1.3 g, 4.5 mmol), and dry DMF (2.0 mL) were placed in a dry sealed ampule. After the solution was degassed by three freeze–evacuate–thaw cycles, the polymerization was conducted at 60 °C for 20 h. The reaction mixture was purified by two-step precipitation into acetone and isolated by filtration to obtain poly[1-methyl-3-(2-methacryloyloxy propylimidazolium bromine)] (PMMPImB), as a light pink powder (0.5 g, 38%).

Synthesis of PMMPImB-@-Au NPs

For the synthesis of PMMPImB-@-Au NPs, PMMPImB terminated with thiol end groups was prepared by the reduction of a dithioester-terminated PMMPImB with hydrazine. To a 100 mL round-bottom flask equipped with a magnetic stirring bar, 0.3 g of dithioester-terminated PMMPImB was dissolved in 30 mL of dry alcohol; 1 mL of 1.0 M aqueous hydrazine solution was added with vigorous stirring. The reaction mixture was stirred at room temperature for more than 3 days and eventually precipitated in THF to yield SH-PMMPImB. SH-PMMPImB aqueous solution (0.002 g mL⁻¹) was added in 100 mL of 1 mM HAuCl₄ and the molar ratio of polymer to gold was ultimately controlled at 1 : 10. The mixed solution was then added dropwise in 2 mL of 0.1 M BTBA. After 3 days of equilibrium at room temperature, the reaction solution was centrifuged at 15 000 rpm for 1.5 h at room temperature, and the supernatant was removed. This process was repeated three times to remove free polymer chains that were not conjugated to gold nanoparticles. Finally, the solid concentrate was re-dispersed in water at pH 7, and the solution of PMMPImB-@-Au NPs was obtained.

Assembly of PMMPImB-@-Au NPs

The stock solution of 0.02 M HPO₄²⁻ and other anionic solutions were prepared. The assembly of PMMPImB-@-Au NPs was performed at room temperature. HPO₄²⁻ (5 μL) was successively added into 5 mL PMMPImB-@-Au NPs solution and monitored by UV spectroscopy after 30 min of incubation. In the presence of HPO₄²⁻, the solution colour gradually turned from wine to purple and blue. The addition was stopped when the second SPR band remained unchanged, indicating the assembly was finished. Other divalent and polyvalent anions, including PO₄³⁻, P₂O₇⁴⁻, SO₄²⁻, and CO₃²⁻, and monovalent anions, such as H₂PO₄⁻, HCO₃⁻, HSO₄⁻, BF₄⁻, NO₃⁻ and PF₆⁻, were tested under the same conditions.

Characterization

The molecular weight distribution (M_w/M_n) of the polymer was characterized with a Waters 600E gel permeation chromatography (GPC) analysis system equipped with a waters StyragelHT column with poly(ethylene oxide) as the calibration standard and water (with 8.5 g L⁻¹ NaNO₃) as the eluent (flow rate: 0.4 mL min⁻¹). Dynamic laser scattering (DLS) measurement was performed using a laser light scattering spectrometer (BI-200SM) equipped with a digital correlator (BI-9000AT) at 532 nm at room temperature. Transmission electron microscopy (TEM) measurements were conducted using a JEM-2100 electron microscopy at an accelerating voltage of 200 kV; a small drop of hybrid micellar solution was deposited onto a carbon-coated copper EM grid and dried at room temperature under atmospheric pressure. The UV-vis spectra were recorded on a Cary 50 Bio UV-Visible Spectrophotometer (Varian, USA), equipped with two silicon diode detectors and a xenon flash lamp. ¹H-NMR spectroscopy was conducted on a Bruker AV300 spectrometer. Zeta-potentials were measured using a temperature-controlled Zetasizer 2000 (Malvern Instruments. Ltd.).

3. Results and discussion

Characterization of PMMPImB

The PILs with controlled molecular weight and low polydistributions have been widely reported by RAFT polymerization due to their special ability for the polymerization of ionic monomers.⁴⁸ In this case, PMMPImB is synthesized by the RAFT polymerization of MMPImB with CPAD as chain transit agent. The ¹H NMR spectrum of PMMPImB in DMSO is displayed in Fig. 1A. From Fig. 1A, the presence of characteristic signals at δ = 9.56 (N–CH=N, b), 7.87–8.14 (N–CH=CH–N, c and d), 4.31 (Im–CH₂–CH₂, f), 4.12 (O–CH₂, g), 3.75 (Im–CH₃, e), 2.19 (CH₂–CH₂–CH₂, h), is ascribed to the pendant groups of PMMPImB. In addition, the signal at δ = 7.1, and 7.4 ppm (a) is the characteristic absorption peak of dithiobenzoyl groups located at the PMMPImB chain end. Thus, the degree of polymerization (DP) of PMMPImB is 24 calculated by the integral area ratio of b and a, and the molecular weight is calculated to be 6450. The molecular weight distribution of PMMPImB was characterized by GPC (Fig. 1B) in water (with 8.5 g L⁻¹ NaNO₃) as the eluent using poly(ethylene oxide) as the calibration standard. The polydispersity index of PMMPImB is 1.17.

Fig. 1 (A) ¹H NMR spectrum in DMSO and (B) GPC traces for PMMPImB in 8.5 g L⁻¹ NaNO₃ aqueous solution of PMMPImB at room temperature.

The characterization of PMMPImB-@-Au NPs

The well-defined polymer synthesized by RAFT has the dithioester or trithiocarbonate end groups, which can easily be converted to thiol, and it can then yield polymer monolayer protected Au NPs due to the strong affinity of sulfur for gold. The common reductive agents, including sodium borohydride and sodium citrate, have been confirmed by us⁴⁸ to introduce a negative charge on the periphery of the Au NPs, which has a strong electrostatic absorption with the PMMPImB chain. As a nonionic reductive agent, BTBA has been reported for preparing cationic lipid-coated Au NPs;⁴⁹ however, a complicated emulsification/solvent evaporation method was used to transfer the particles from the organic solvent to water. Here, BTBA is directly used to reduce the mixture of chloroauric acid and PMMPImb. The covalent bonding PMMPImb on the surface of the Au NPs can easily be achieved to form the core–shell composite through the techniques of the “grafting to” approach. The resulting solution is subjected to three centrifugation/washing/redispersing cycles to remove unbound polymers.

The UV-vis spectroscopy and TEM were first used to characterize the core/shell hybrid particles. As shown in Fig. 2A, the characteristic SPR band of Au NPs is observed in the spectrum at approximately 530 nm. From the results of TEM, the inset image in Fig. 2A, it can be inferred that the particles are well dispersed and have Au NPs core with a mean diameter of approximately 8 nm. The characterization of TGA and DLS can directly provide the information of the core–shell structure, as shown in Fig. 2B. From Fig. 2B, the PMMPImb is found to decompose in the temperature range from 262 to 450 °C. Thus, the total weight percentage of grafted polymers could be determined, and the graft density of polymer chains could be calculated as follows:

where W_polymer is the percentage of weight loss according to the decomposition of the polymer, P is the density of gold (19.32 g cm⁻³), V_particles is the volume of the gold nanoparticle calculated from the radius measured by TEM, N_A is Avogadro's number, M_polymer is the molecular weight of polymer and S_particle is the surface area of the gold nanoparticle. The calculated grafting density of PMMPImb on the surface of each of the AuNPs is approximately 0.85 chains per nm². The hydrodynamic diameter and distribution of PMMPImB-@-Au NPs was measured by DLS. As shown in the inset image of Fig. 2(B), the average hydrodynamic diameter (D_h) of the particles is approximately 31 nm and the size distribution range is 20–46 nm. From the results of TEM and DLS, the mean length of the shell is approximately 11 nm.

Fig. 2 UV-vis spectra of PMMPImB-@-Au NPs (A) and TGA analysis (B) of PMMPImB (a) and PMMPImB-@-Au NPs (b), and of the PMMPImB-@-Au NPs, where the inset images in A and B are the TEM image of PMMPImB-@-Au NPs and the DLS spectra of PMMPImB-@-Au NPs.

Fig. 3 shows the evolution of SPR of PMMPImB-@-Au NPs after the addition of HPO₄²⁻ at increasing concentrations. In this case, the spectra are measured after 30 min of incubation to ensure the saturation of the assembly process. Upon the addition of HPO₄²⁻, the intensity of the transverse band at 530 nm declines. The second new SPR band in the higher wavelength region gradually appears, indicating the formation of aggregated nanostructures. The appearance of a long-wavelength plasmon band is caused by the coupling of the plasmon resonance of neighbouring Au NPs. The intensity of the long-wavelength band increases with the increase in ion concentration and finally reaches a plateau at 640 nm at 35 μL of HPO₄²⁻. The longitudinal mode of plasmon oscillation is excited along with the formation of a longer chain. Moreover, these spectral changes are also accompanied by a clear colour change of PMMPImB-@-Au NPs from wine red to blue (insets in Fig. 3).

Fig. 3 UV-vis spectra and images of PMMPImB-@-Au NPs with the addition of different volumes of HPO₄²⁻.

The TEM images of PMMPImB-@-Au NPs in the presence of different concentrations of HPO₄²⁻ clearly confirm the self-assembled process of gold nanochain formation. When HPO₄²⁻ was added to the solution of PMMPImB-@-Au NPs, the single HPO₄²⁻ ions were randomly associated with two imidazolium groups on the periphery of the particle via electrostatic dipole interactions. The groups can be from single or neighbouring particles. Therefore, at low amounts of HPO₄²⁻ (5 μL and 10 μL), single particle, two particles and short chains of PMMPImB-@-Au NPs coexist in the image (Fig. 4a and b). When the amount of HPO₄²⁻ is increased to 25 μL, the conjunction among particles or short chains can induce the formation of longer chains (Fig. 4c). The long chains are a random walk with a branched structure due to random interaction and secondary assembly. In addition, the high-resolution TEM image of adjacent nanoparticles (Fig. 4c, inset) clearly revealed the nanochain formation through the interaction of their shell via HPO₄²⁻. When the amount of HPO₄²⁻ is further increased, as shown in Fig. 4d, the short chains are further assembled with the direction of either length or width and form longer chains and partial 2D structure. However, a small number of aggregated Au NPs are also found in Fig. 4d. To elucidate the mechanism of their formation, we quantitatively analyzed the impact of the concentration of PMMPImB-@-Au NPs on the structure of the nanochain, illustrated in the ESI.† Based on the results of the TEM (Fig. 1s†) and UV-vis spectra (Fig. 2s†), it can be inferred that the concentration of PMMPImB-@-Au NPs is the main reason that causes the uncontrolled aggregation of Au nanoparticles. Reducing the concentration of PMMPImB-@-Au NPs can suppress the formation of such aggregation, but it would produce shorter chain and weaker absorbance in the long-wavelength plasmon band, which is disadvantageous for practical applications. Therefore, the present concentration is feasible.

Fig. 4 TEM images of PMMPImB-@-Au NPs in the presence of 5 μL (a), 10 μL (b), 25 μL (c) and 35 μL (d) of HPO₄²⁻.

The electrostatic dipole interaction only served as the linker among Au NPs. The electrostatic repulsive interaction from the particle surface charge favored the end-on alignment of particles to a chain.^50,51 The formation of the nanochain was driven by a balance between two opposite interactions.⁵¹ The change of two interactions can be analysed by the measurement of particle surface potentials. The zeta-potentials of PMMPImB-@-Au NPs steadily decline from the original value of 24.8 to 14.2 mV after the addition of 35 μL of HPO₄²⁻ (Fig. 5). With an increase in the amount of HPO₄²⁻, the decreased positive zeta-potential is caused by the occurrence of electrostatic dipole interactions. At high HPO₄²⁻ concentrations, the lower electrostatic repulsive interactions will make the further assembly of particles to form a close-packed structure.

Fig. 5 Zeta-potentials of PMMPImB-@-Au NPs in the presence of 5 μL (a), 10 μL (b), 25 μL (c) and 35 μL (d) HPO₄²⁻.

The cyanide has the ability to dissolve Au NPs, and this process can be used to observe the remaining structure of the polyelectrolyte complexes.⁵² In this case, the electrostatic conjugation of HPO₄²⁻ and the imidazolium groups on the PIL shell is linked to Au NPs to form a nanochain, where the shell interaction between coupling particles can be studied after the removal of Au cores. The solution of the plasmonic nanochain was first fixed by replenishing 35 μm of HPO₄²⁻, then adding 20 μm of 0.1 M KCN to etch the AuNPs. After being dialysed against needless ions, the solution turned colourless and was directly deposited onto a carbon-coated copper grid for observation under TEM. The TEM images of the assemblies of the removed Au NPs are shown in Fig. 6. From Fig. 6a, the close-packed nanocapsules are clearly found upon the dissolution of the Au cores. The close interaction among nanocapsules can be observed in Fig. 6b, a local magnified image of Fig. 6a. The single nanocapsule has a mean diameter of approximately 19 nm. This dimension is lower than the DLS result of PMMPImB-@-Au NPs, indicating the shrinkage of the shell cross-linked by more HPO₄²⁻. The HPO₄²⁻ binding with two imidazoliums on PMMPImB is first induced for the formation of the chain-like structure, and then further interaction to develop a close-packed nanochain and a cross-linked shell layer. The connected nanocapsules were subsequently obtained after the removal of Au cores.

Fig. 6 TEM images of chain-like nanostructures after the dissolution of gold cores by KCN with the chain-like nanostructure in the presence of 70 μL of HPO₄²⁻.

The size of the capping material determines the interparticle distance, and thus affects the coupling properties of the assemblies.⁵² In this work, we prepared different chain lengths of PMMPImB capped Au NPs via RAFT polymerization and then used the “grafting to” approach. Fig. 7 shows the UV-vis spectra of PMMPImB-@-Au NPs at the different chain lengths of capping PMMPImB after the addition of 35 μL HPO₄²⁻. The repeated unit of PMMPImB is 82, 56, 35 and 24. From Fig. 7c and d, the UV-vis spectra of PMMPImB-@-Au NPs at 24 and 35 of the repeated units present a clear second long-wavelength plasmon band at 640 and 625 nm, respectively. However, when the repeated units increase to 56, the long-wavelength plasmon band weakens and is located at 614 nm (Fig. 7b). The second plasmon band, which has almost disappeared, is found in the UV-vis spectrum of PMMPImB-@-Au NPs at 82 of the repeated units (Fig. 7a), suggesting that the coupling property of adjacent Au NPs hardly occurred at the large interparticle distance. These results confirm that the coupling property of the plasmonic nanochain can be tuned by controlling the size of the capping polymer, which can be achieved through RAFT polymerization.

Fig. 7 UV-vis spectra of PMMPImB-@-Au NPs with the different repeated units of PMMPImB in the presence of 35 μL of HPO₄²⁻, where a, b, c and d are 82, 56, 31 and 24 units, respectively.

To further analyse the linear self-assembly mechanism, we compared the UV-vis spectra of PMMPImB-@-Au NPs in the presence of the same concentrations of divalent anions, polyvalent anions, and monovalent anions, as shown in Fig. 8. Fig. 8a shows the UV-vis spectra of the PMMPImB-@-Au NPs after adding some familiar divalent or polyvalent anions including HPO₄²⁻, SO₄²⁻, CO₃²⁻, PO₄³⁻ and P₂O₇⁴⁻. In Fig. 8a, in addition to the transverse SPR band at 530 nm, the longitudinal SPR band at 630–640 nm is also found in these spectra, indicating the chain-like Au NPs formation. The divalent and polyvalent anions can also be regarded as the linkers among PMMPImB-@-Au NPs and induce a chain-like arrangement of the particles. The longitudinal SPRs reveal a close position even though the longitudinal SPRs are under the action of different anions. The results show that the different anions do not affect the interparticle distance due to the sizes of molecules being smaller than the polymer. Fig. 8b shows the UV-vis spectra of PMMPImB-@-Au NPs after the addition of some corresponding monovalent anions and hydrophobic anions of PILs. In Fig. 8b, only the transverse SPRs are found, indicating that these monovalent anions lack the ability to induce the formation of chain-like Au NPs. The little red-shifted SPRs are found in the UV-vis spectra of PMMPImB-@-Au NPs in the presence of HCO₃⁻, H₂PO₄⁻, HSO₄⁻, PF₆⁻ and BF₄⁻, suggesting the formation of the aggregated PMMPImB-@-Au NPs. This isotropous aggregation is caused by the hydrophilic–hydrophobic transition of PILs due to anion responsiveness. This result further confirms that the chain-like Au NPs are induced by electrostatic dipole interaction, instead of hydrophobic interaction or hydrogen-bonding interaction.

Fig. 8 UV-vis spectra of the PMMPImB-@-Au NPs after the addition of 35 μL of different divalent and polyvalent (a) and monovalent (b) anions.

The formation of gold nanochains induced by SO₄²⁻, CO₃²⁻, PO₄³⁻ and P₂O₇⁴⁻ is also confirmed by the TEM images, shown in Fig. 9. From Fig. 9a and b, the solutions of PMMPImB-@-Au NPs in the presence of 35 μL of SO₄²⁻ and CO₃²⁻ reveal a chain-like structure similar to HPO₄²⁻ at the same concentration. However, trivalent and tetravalent anions, such as PO₄³⁻ and P₂O₇⁴⁻ (Fig. 9c and d), represent the larger assemblies with increasing particle number and chain length, comprised of bifurcations and enclosed loops of outgrowths. In particular, for the image of P₂O₇⁴⁻, the structure shows the chains with two particles arranged side by side with densely compacted nanoparticle agglomerates. Although the same number of anions was used to drive the assembly of PMMPImB-@-Au NPs, polyvalent anions display more complex chain-like arrays because of the following reasons: first, the arrangement of particles is random and formed flexible chain; second, the polyvalent anions provide more active sites to conjugate with imidazolium groups.

Fig. 9 TEM image of PMMPImB-@-Au NPs after the addition of 35 μL of CO₃²⁻ (a), SO₄²⁻ (b), PO₃³⁻ (c) and P₂O₄⁴⁻ (d).

In particular, all of the solutions of assemblies induced by HPO₄²⁻, SO₄²⁻, CO₃²⁻, PO₄³⁻ and P₂O₇⁴⁻ are generally stable under some harsh conditions, including acidic, alkali and in the presence of metal ions such as Na⁺, K⁺, Ba²⁺, and Ca²⁺. The colour and UV-vis spectra show no change after the addition of such aqueous solutions. The results indicate that the imidazolium cation has a higher binding affinity to these anions than H⁺, OH⁻ and some metal ions. In addition, the study about time-dependent optical spectra (Fig. S3†) reveals a rapid self-assembled process and further demonstrates this type of higher binding affinity.

4. Conclusions

In summary, a new core–shell plasmonic particle, PILs capped Au NPs, has been prepared and further fabricated into 1D array nanostructures in aqueous solutions. The divalent, trivalent, and tetravalent anions could serve as the linkers of neighbouring particles via electrostatic dipole interactions for arranging the isotropic particles into nanochains. The process has been confirmed by the increase in the intensity of longitudinal plasmon band in the near-infrared absorption region, and it was also observed with TEM. In this study, the position of the longitudinal plasmon band was unrelated to the type of anion at fixed polymer lengths but was tuned by the change in the polymer length. The nanochain that was formed could be used for preparing connected nanocapsule materials, and could also be used for discriminating the polyvalent from its corresponding monovalent anion via different colours and spectral changes. Considering their high stability in water solutions containing acid, alkali, and metal ions and combining the intriguing optics and surface properties, the chain-like nanostructure based on PILs capped Au NPs are a subject for further investigation for biomedical applications such as cell-imaging, photo-thermal imaging, and gene therapy for cancer treatment and controlled release.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51103035 and 51403055) and Scientific & Technological Project of Henan Province (no. 132102310422).

References

1. Z. Nie, A. Petukhova and E. Kumacheva, Nat. Nanotechnol., 2010, 5, 15–25.
2. A. H. Khan, S. Maji, S. Chakraborty, N. B. Manik and S. Acharya, RSC Adv., 2012, 2, 186–191.
3. C. Mondal, A. H. Khan, B. Das, S. Acharya and S. Sengupta, Sci. Rep., 2013, 3, 6.
4. J. A. Fan, C. H. Wu, K. Bao, J. M. Bao, R. Bardhan, N. J. Halas, V. N. Manoharan, P. Nordlander, G. Shvets and F. Capasso, Science, 2010, 328, 1135–1138.
5. H. Wang, D. W. Brandl, P. Nordlander and N. J. Halas, Acc. Chem. Res., 2007, 40, 53–62.
6. Z. Deng, Y. Tian, S.-H. Lee, A. E. Ribbe and C. Mao, Angew. Chem., Int. Ed., 2005, 44, 3582–3585.
7. B. Pelaz, S. Jaber, D. J. de Aberasturi, V. Wulf, T. Aida, J. M. de la Fuente, J. Feldmann, H. E. Gaub, L. Josephson, C. R. Kagan, N. A. Kotov, L. M. Liz-Marzan, H. Mattoussi, P. Mulvaney, C. B. Murray, A. L. Rogach, P. S. Weiss, I. Willner and W. J. Parak, ACS Nano, 2012, 6, 8468–8483.
8. A. N. Shipway, E. Katz and I. Willner, ChemPhysChem, 2000, 1, 18–52.
9. E. Katz and I. Willner, Angew. Chem., Int. Ed., 2004, 43, 6042–6108.
10. Z. Zhu, W. Liu, Z. Li, B. Han, Y. Zhou, Y. Gao and Z. Tang, ACS Nano, 2012, 6, 2326–2332.
11. X. Zan, S. Feng, E. Balizan, Y. Lin and Q. Wang, ACS Nano, 2013, 7, 8385–8396.
12. Y. Ohya, N. Miyoshi, M. Hashizume, T. Tamaki, T. Uehara, S. Shingubara and A. Kuzuya, Small, 2012, 8, 2335–2340.
13. N. M. Sangeetha, C. Blanck, N. Thi Thanh Tam, C. Contal and P. J. Mesini, ACS Nano, 2012, 6, 8498–8507.
14. J. Sharma, R. Chhabra, A. Cheng, J. Brownell, Y. Liu and H. Yan, Science, 2009, 323, 112–116.
15. N. J. Halas, S. Lal, W.-S. Chang, S. Link and P. Nordlander, Chem. Rev., 2011, 111, 3913–3961.
16. N. L. Rosi and C. A. Mirkin, Chem. Rev., 2005, 105, 1547–1562.
17. P. K. Jain, X. H. Huang, I. H. El-Sayed and M. A. El-Sayed, Acc. Chem. Res., 2008, 41, 1578–1586.
18. D. Lee, Y.-J. Choe, Y. S. Choi, G. Bhak, J. Lee and S. R. Paik, Angew. Chem., Int. Ed., 2011, 50, 1332–1337.
19. F. Favier, E. C. Walter, M. P. Zach, T. Benter and R. M. Penner, Science, 2001, 293, 2227–2231.
20. K. K. Zhang, M. Jiang and D. Y. Chen, Angew. Chem., Int. Ed., 2012, 51, 8744–8747.
21. M. E. Stewart, C. R. Anderton, L. B. Thompson, J. Maria, S. K. Gray, J. A. Rogers and R. G. Nuzzo, Chem. Rev., 2008, 108, 494–521.
22. K. Liu, A. Ahmed, S. Chung, K. Sugikawa, G. Wu, Z. Nie, R. Gordon and E. Kumacheva, ACS Nano, 2013, 7, 5901–5910.
23. L. Wang, L. Xu, H. Kuang, C. Xu and N. A. Kotov, Acc. Chem. Res., 2012, 45, 1916–1926.
24. A. J. Amali, S. Singh, N. Rangaraj, D. Patra and R. K. Rana, Chem. Commun., 2012, 48, 856–858.
25. H. Wu, F. Bai, Z. Sun, R. E. Haddad, D. M. Boye, Z. Wang and H. Fan, Angew. Chem., Int. Ed., 2010, 49, 8431–8434.
26. M. Li, S. Johnson, H. Guo, E. Dujardin and S. Mann, Adv. Funct. Mater., 2011, 21, 851–859.
27. M. Grzelczak, J. Vermant, E. M. Furst and L. M. Liz-Marzan, ACS Nano, 2010, 4, 3591–3605.
28. J. A. Fan, C. Wu, K. Bao, J. Bao, R. Bardhan, N. J. Halas, V. N. Manoharan, P. Nordlander, G. Shvets and F. Capasso, Science, 2010, 328, 1135–1138.
29. A. Guerrero-Martinez, M. Grzelczak and L. M. Liz-Marzan, ACS Nano, 2012, 6, 3655–3662.
30. M. M. Abul Kashem, D. Patra, J. Perlich, A. Rothkirch, A. Buffet, S. V. Roth, V. M. Rotello and P. Mueller-Buschbaum, ACS Macro Lett., 2012, 1, 396–399.
31. R. R. Bhattacharjee and T. K. Mandal, J. Colloid Interface Sci., 2007, 307, 288–295.
32. J. Li, C. Han, W. Wu, S. Zhang, J. Guo and H. Zhou, New J. Chem., 2014, 38, 717–722.
33. R. Oren, Z. Liang, J. S. Barnard, S. C. Warren, U. Wiesner and W. T. S. Huck, J. Am. Chem. Soc., 2009, 131, 1670–1671.
34. M. S. Yavuz, G. C. Jensen, D. P. Penaloza, T. A. P. Seery, S. A. Pendergraph, J. F. Rusling and G. A. Sotzing, Langmuir, 2009, 25, 13120–13124.
35. M. Beija, J.-D. Marty and M. Destarac, Chem. Commun., 2011, 47, 2826–2828.
36. S. Chakraborty, S. W. Bishnoi and V. H. Perez-Luna, J. Phys. Chem. C, 2010, 114, 5947–5955.
37. X. Qian, J. Li and S. Nie, J. Am. Chem. Soc., 2009, 131, 7540–7541.
38. A. Housni, Y. Zhao and Y. Zhao, Langmuir, 2010, 26, 12366–12370.
39. J. Liu, C. Detrembleur, M.-C. De Pauw-Gillet, S. Mornet, E. Duguet and C. Jerome, Polym. Chem., 2014, 5, 799–813.
40. D. Smith, A. C. Holley and C. L. McCormick, Polym. Chem., 2011, 2, 1428–1441.
41. K. L. Hamner, C. M. Alexander, K. Coopersmith, D. Reishofer, C. Provenza and M. M. Maye, ACS Nano, 2013, 7, 7011–7020.
42. J. He, P. Zhang, T. Babu, Y. Liu, J. Gong and Z. Nie, Chem. Commun., 2013, 49, 576–578.
43. J. He, X. Huang, Y.-C. Li, Y. Liu, T. Babu, M. A. Aronova, S. Wang, Z. Lu, X. Chen and Z. Nie, J. Am. Chem. Soc., 2013, 135, 7974–7984.
44. E. B. Anderson and T. E. Long, Polymer, 2010, 51, 2447–2454.
45. D. Mecerreyes, Prog. Polym. Sci., 2011, 36, 1629–1648.
46. B. Yu, F. Zhou, Y. Bo, X. Hou and W. Liu, Electrochem. Commun., 2007, 9, 1749–1754.
47. S.-i. Yusa, M. Sugahara, T. Endo and Y. Morishima, Langmuir, 2009, 25, 5258–5265.
48. J. Li, J. Liang, W. Wu, S. Zhang, K. Zhang and H. Zhou, New J. Chem., 2014, 38, 2508–2513.
49. W. H. Kong, K. H. Bae, S. D. Jo, J. S. Kim and T. G. Park, Pharmaceut. Res., 2012, 29, 362–374.
50. X. Shen, L. Chen, D. Li, L. Zhu, H. Wang, C. Liu, Y. Wang, Q. Xiong and H. Chen, ACS Nano, 2011, 5, 8426–8433.
51. C. Fang, Y. Fan, J. M. Kong, Z. Q. Gao and N. Balasubramanian, Appl. Phys. Lett., 2008, 92, 263108.
52. P. Kekicheff, G. F. Schneider and G. Decher, Langmuir, 2013, 29, 10713–10726.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14625e

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