Facile reversible LSPR tuning through additive-induced self-aggregation and dissemination of Ag NPs: role of cyclodextrins and surfactants

Abstract

Tuning the optoelectronic properties by controlling coupled surface plasmon modes in metal nanomaterials is a major challenge. Several methods have been developed for localized surface plasmon resonance (LSPR) tuning, which are generally provoked by the self-assembly of a certain molecule (e.g. protein or viologen) that cause aggregation of nanoparticles. Here, we have developed a new, simple method that is applied on dilute cetyltrimethylammonium bromide (CTAB)-stabilized Ag NPs for reversible LSPR tuning through additive-induced formation and subsequent dispersion of self-aggregates. Addition of cyclodextrin (α- or β-) results in a decrease of the surface charge of NPs by taking away CTAB from the nanoparticle surface through formation of an inclusion complex. The slow formation of self-aggregates of Ag NPs is due to the gradual decrease in surface charge, which results in a large red-shifting of the LSPR band (436 nm to 537 nm). Subsequent addition of different types of surfactants alters the surface charge by re-attachment of stabilizer and results in dispersion of self-aggregates with blue-shifting of the LSPR band (537 nm to 420 nm). This efficient formation and break down of self-aggregates are monitored by dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements and are correlated with the alteration of the plasmonic absorption band. Variation in surface charge of Ag NPs is followed by zeta-potential measurements. This easy approach to control the plasmon absorption position can be very significant in applications such as sensing and optoelectronic devices.

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1. Introduction

During the past two decades there has been a growing research interest in the field of various metal nanomaterials due to their unique optical, electrical, and magnetic properties for broad applications in optoelectronics, catalysis, sensors, and therapeutics.^1–5 The appearance of localized surface plasmon resonance (LSPR) is one of the most interesting phenomena of metal nanomaterials.⁶ The electric field of electromagnetic radiation causes a collective coherent oscillation of the conductive electrons in nanomaterials at the metal and dielectric interface. Such oscillating resonance, known as LSPR, appears in the visible region for noble metals such as silver and gold. Control of light at the nanoscale using surface plasmons⁷ encompasses various unique optical phenomena such as enhancement of localized electromagnetic (EM) field at a nanostructured metal surface,⁸ extraordinary sensitivity of the LSPR to an external medium,⁹ and high transmission through sub-wavelength apertures in thin metal films.^10,11 LSPR is of enormous interest due to its potential applications in plasmonic circuits,^12,13 photovoltaics^14,15 and chemical/biological sensors.¹⁶ Various factors are responsible for LSPR of metal nanostructures, such as composition, size, shape, dielectric ambient and proximity to other nanoparticles (plasmon coupling).^17,18

Tuning of LSPR along with the plasmonic properties of metal nanostructures and their assemblies is a growing challenge because of the potential applications in sensing, optoelectronics and photonics. Formation of aggregates of the nanostructures, by modification of solution conditions or the nanoparticle surface,¹⁹ is a reliable way for LSPR tuning as the plasmon coupling strength decays exponentially with separation.²⁰ The distance of separation between the nanostructures is highly dependent on the extent of aggregation.

The phenomenon of aggregation of the particles in a colloid solution can be brought about by various means, e.g., (i) upon addition of a “self-assembling” molecule or (ii) modulation of surface charge of the nanoparticles. The aggregation process starts with a color change, which is eventually followed by the precipitation of the nanoparticles. This type of precipitation, due to over-aggregation, is a nuisance to the scientist who would like to have a stable aggregated colloid for various applications such as surface enhanced Raman scattering (SERS)^21,22 or surface plasmon spectroscopy (SPS).²³ Several possibilities have been explored to obtain a stable aggregated colloid in a controlled way. Organic molecules, macromolecular scaffolds and biomolecules such as polymers,²⁴ dendrimers,²⁵ multi-dentate thioethers²⁶ and proteins²⁷ are used as self-assembling molecules for the formation of stable aggregated nanosols. Poly(amidoamine) (PAMAM) dendrimer has been used to tune Au-nanoparticle interparticle spacing by varying the size of the dendrimers.²⁵ Alternatively, lysozyme has been employed as a model protein to control the interparticle spacing by changing the molar ratio of protein to Au-nanoparticles or by controlling the assembly temperature.²⁷ Previous literature reports reveal that carboxylate-functionalized nanosols or carboxylate-containing-DNA-functionalized colloids display aggregation behavior as a function of pH and metal ion concentration.^28–30 At high pH (complete deprotonation of carboxyl group), the particles in the colloid remain separated; however, lowering of the pH leads to aggregation due to protonation of some carboxylic functional groups, allowing interactions between the particles. The extent of aggregation can often be controlled by reaching the appropriate pH in the colloidal solution. Binding of heavy metals with carboxylate groups lowers the surface charge of the nanoparticles causing aggregation. This type of aggregation process can be employed to monitor the heavy metal ion level for water treatment.^29,30 A similar type of controlled and stable aggregates of Au-nanoparticles has been achieved by varying the pH of a poly(4-vinylpyridine) (P4VP) polymer film.³¹ The aggregation and dispersion of Au-nanoparticles occur because of the pH-responsive coiled and extended states of the polymer chain. Moreover, reversible aggregation of DNA-coated colloids has been observed for which the temperature of the single particle–aggregate transition is indicative of the degree of DNA complementarity.³² However, in most of the cases the self-assembling molecules insert themselves between the nanoparticles during the aggregation process, i.e., one extra molecule remains incorporated within the nano-assemblies, which can cause complications for further studies using these aggregated nanoparticles.

Herein, for the first time, we have demonstrated a facile strategy for the reversible tunability of LSPR through additive-induced formation and subsequent dispersion of self-aggregates of dilute cetyltrimethylammonium bromide (CTAB)-stabilized silver nanoparticles (Ag NPs). The LSPR of Ag NPs gets red-shifted by the addition of cyclodextrin (CDx) to the silver colloid, possibly due to self-aggregation of the nanoparticles. Addition of surfactants to a cyclodextrin pre-treated Ag NP sample can stop and also lead to blue-shifting of the LSPR band through break down of self-aggregates. NPs get stabilized and dispersed in solution as a colloid due to the surface charge. In the case of our NPs, the surface charge (causing a “ζ-potential”) is provided by the stabilizer surfactant molecules adhering to the surface. Thus, the decrease of surface charge, as a result of the removal of those surfactants, lowers the ζ-potential leading to NP-aggregation. Re-attachment of ionic surfactant on the surface can increase the ζ-potential causing dissemination of NP. Therefore, the well-controlled surface charge plays a leading role in the formation and subsequent dispersion of self-aggregates of Ag NPs. Moreover, surfactants of different charge-type can lead to different inherent properties of nanoparticles such as structural stability, adsorption characteristics, surface charge, and reshaping and redistribution of particle morphology. Hence, this method is an efficient and cost-effective tool for the generation of stable nano-aggregates without insertion of any self-assembling molecule, which could pave the way for creation of new materials for applicative reasons.

Here we use surfactants having different charge-type head groups (namely, positive, negative and neutral) for the reversal of the LSPR position (blue shifting), which ought to modify the surface charge of NPs, and this will be evident from zeta-potential measurements. This well-tuned surface charge of nanoparticles plays a dominant role in binding, disrupting, and penetrating through many proteins, DNA, and cell-membranes as the interactions of these systems with nanoparticles are predominantly electrostatic in nature. The interaction of HSA with the nanoparticles is also predominantly electrostatic, and interestingly, the protein concentration for stabilization of the conjugates decreases when the overall negative charge on the nanoparticle surface increases.³³ Positive surface charge on nanoparticles is efficient for absorption, whereas negative surface charge on nanoparticles is found to be efficient for desorption of DNA.³⁴ Moreover, positively charged nanoparticles can bind, disrupt and penetrate cell membranes to a large extent, while neutral, negative, and zwitterionic NPs have negligible effects.^35,36 It has been reported that hydroxyapatite (HAP) nanoparticle-induced aggregation of red blood cells (RBCs) occurs via an electrostatic interaction between positively charged binding sites on the HAP surface and negatively charged groups on the surface of the RBCs.³⁷ The surface charge of nanoparticles can be crucial for drug/fluorescent-probe carrier systems because many proteins, DNA, and cell-membrane surfaces are slightly anionic.³⁸

2. Materials and methods

2.1 Materials

Purest grade carmoisine, silver nitrate (AgNO₃), cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CpCl), Triton X-100 (Tx-100), sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), and α-, β- and γ-cyclodextrin are purchased from Sigma Aldrich. NaOH (AR grade) is purchased from Merck. The experiment is performed in deionized triply distilled water. All glassware is cleaned thoroughly by nitric acid and freshly prepared chromic acid, rinsed thoroughly with distilled water and acetone, and then dried in an oven.

2.2. Instrumentation

Photochemical reactions are performed by using a xenon lamp from Newport, model 66902 (300 watts), in a photo-reactor made up of a 100 ml round bottom flask (made with borosilicate glass) with a magnetic stirrer. Light luminous flux per unit area is measured by using a Lutron LX-107HA digital light meter, and the synthesis is performed at a light luminous flux per unit area of 50000 (±100) Lux. The pH measurements are carried out on a Eutech-510 ion pH-meter, which is pre-calibrated with standard pH buffer tablets. Electronic absorption spectra are recorded with a UV-2450 (Shimadzu) absorption spectrophotometer against a reference solvent. Transmission electron microscopy (TEM) images are acquired using an FEI-Tecnai G2 20 s-Twin with an operating voltage of 200 kV. Measurement of particle size distribution through dynamic light scattering (DLS) and surface charge of the nanoparticles by zeta potential is performed with a Malvern Nano ZS instrument employing a 4 mW He–Ne laser operating at a wavelength of 632.8 nm and an avalanche photodiode (APD) detector.

2.3. Synthesis of Ag nanostructures

Ag NP is prepared by our previously reported photochemical method.³⁹ In a typical procedure, 50 ml of the reaction mixture at pH 9.0 (made by adding drops of aqueous NaOH of 0.1 M concentration) in a round bottom flask, containing dilute carmoisine (10 μM), CTAB and AgNO₃ at a mole ratio of 1 : 5 : 5, are irradiated with broad band visible light from the xenon source using a light luminous flux per unit area of 50 000 (±100) Lux at 25 °C for 180 minutes with constant stirring using a magnetic stirrer. The characteristic pale yellow colored silver sol is characterized by UV-Vis spectroscopy and TEM imaging.

2.4 UV-Vis Spectroscopy, DLS, TEM and zeta-potential measurements

The additive-induced tuning of the LSPR band, hydrodynamic radius, morphology and surface charge are monitored by UV-Vis spectroscopy, DLS, TEM and zeta-potential measurements, respectively, for samples obtained by incubating the as-synthesized Ag NP solution for one hour after the addition of 50 μM of cyclodextrin (α-, β- and γ-) to the solution. The same measurements are performed after the addition of various concentrations of surfactants of different charge-type (CTAB, CpCl, Tx-100, SDS and SDBS) to the cyclodextrin-pretreated Ag NP sample. All measurements are taken at various time intervals after addition of a fixed concentration of CDx (0.05 mM), and the representative spectra and/or images shown correspond to samples for 60 min intervals only. Further, different concentrations of a variety of surfactants are added to the samples, and measurements are repeated.

3. Results and discussion

Extinction spectra of our synthesized CTAB-stabilized silver nanoparticles show the characteristic plasmon absorption band at 436 nm. Fig. 1A and B show the changes in extinction spectra and shifting of the localized surface plasmon resonance (LSPR) peak position with time, respectively, of our as-synthesized Ag NPs in the absence and presence of 50 μM α-, β-, or γ-CDx. After addition of CDx, the extinction spectrum of the Ag NPs shifts to longer wavelength with a concomitant change in color (pale yellow color discharges) with α- and β-CDx, but not for γ-CDx. The significant red shifting of the LSPR band (to 537 nm for α-CDx and 474 nm for β-CDx) with time is attributed to the strong plasmon coupling of the self-aggregated Ag NPs.⁴⁰ This is also evident from DLS studies and TEM images as discussed later.

Fig. 1 (A) UV-Vis extinction spectra and (B) LSPR peak position of Ag NPs in the absence and presence of 50 μM α-, β-, or γ-CDx with time. UV-Vis extinction spectra of (C) α- and (E) β-CDx pre-treated Ag NP samples in the absence and presence of different concentrations of various surfactants. Plots of LSPR peak position of (D) α- and (F) β-CDx pre-treated Ag NP samples in the absence and presence of different concentrations of various surfactants.

The shift in the LSPR band position can be arrested at any stage in time and also can revert back to its initial band position in the presence of different concentrations of surfactants (e.g. CTAB, CpCl, Tx-100, SDS and SDBS). Addition of surfactant leads to blue shifting of the initially CDx-induced red-shifted LSPR band (Fig. 1C and E) with consequent re-appearance of the pale yellow color. This blue-shifting of the LSPR peak position (Fig. 1D and F) is attributed to be caused by breaking of self-aggregates of Ag NPs, as evident from DLS studies and recorded TEM images, and is discussed later. Note that the nature of concentration-dependent back-shifting depends on the charge-type of the head-group of the surfactant used. Addition of a minimum concentration of positively charged surfactants (listed in Table 1) can stop the time-dependent red-shifting of the LSPR band to a certain position (see Table 1), and further addition leads to reversal of the LSPR position (blue-shifting). On the other hand, addition of a minimum concentration of neutral or negatively charged surfactants (as shown in Table 1) leads to sudden blue-shifting of the LSPR band without any stoppage, unlike cationic surfactants.

Table 1 List of minimum concentrations of different surfactants required to stop the CDx-induced LSPR shifting of Ag NPs at any time. The LSPR peak positions at the corresponding surfactant concentrations are also tabulated (any perceptible change in LSPR peak position has not been observed below this minimum concentration)

Surfactant	Charge on the head group	Minimum concentration (mM)	LSPR peak position for α/β-CDx (nm)
CTAB	Positive	0.01	534/473
CpCl	Positive	0.01	536/473
Tx-100	Neutral	2	425/426
SDS	Negative	4	427/428
SDBS	Negative	0.5	425/424

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The DLS technique is utilized to monitor the change in hydrodynamic radius (R_h), and TEM imaging is employed to follow the morphological changes of CTAB-coated Ag NPs after additions of CDx and also for samples with subsequently added surfactants. Fig. 2A and B exhibit gradual increases in hydrodynamic radius with time in the absence and presence of 50 μM α- or β-CDx. The average hydrodynamic radius of our synthesized Ag NPs is ∼142 nm, and after addition of CDx, it increases to ∼827 nm and ∼398 nm for α-CDx and β-CDx, respectively. On the other hand, TEM images reveal that initially our synthesized Ag NPs mostly exist as a distribution of stand-alone particles (Fig. 3A), and after addition of CDx (50 μM) the particles get aggregated (Fig. 3B and C). Thus, the self-aggregation of NPs, as evident from the TEM images and the increase in hydrodynamic radius, appears to be a significant factor in the large red-shifting of the LSPR band with time. The LSPR band resulting from plasmon coupling of the self-aggregated nanoparticles is known as the coupled plasmon band, the peak position of which depends on the extent of aggregation.³¹ The greater extent of self-aggregation of Ag NPs in the presence of α-CDx than in the presence of β-CDx at the same added concentration is accompanied by a larger red-shifting of the LSPR band in the former. The broadening of the LSPR band and DLS band-width due to CDx indicates the polydispersity of the nanoparticle aggregates, which is also evident from TEM images.

Fig. 2 (A) DLS spectra and (B) average hydrodynamic radius (R_h) of Ag NPs in the absence and presence of 50 μM α- or β-CDx with time. DLS spectra of (C) α- and (E) β-CDx pre-treated Ag NP samples in the absence and presence of different concentrations of various surfactants. Plots of average hydrodynamic radius (R_h) of (D) α- and (F) β-CDx pre-treated samples in the absence and presence of different concentrations of various types of surfactants.

Fig. 3 TEM images of Ag NPs in (A) the absence and presence of 50 μM (B) α-CDx and (C) β-CDx at 60 min. Representative TEM images of α-CDx pre-treated Ag NP samples in the presence of (D) 0.8 mM CTAB, (E) 0.8 mM CpCl, (F) 2 mM Tx-100, (G) 4 mM SDS and (H) 0.5 mM SDBS.

The DLS spectra of CDx-induced self-aggregated Ag NP samples exhibit a concentration dependent gradual decrease in hydrodynamic radius with positively charged surfactants as additives, whereas a sudden decrease in hydrodynamic radius is observed with neutral or negatively charged surfactants (Fig. 2C–2F). TEM images of CDx pre-treated Ag NP samples in the absence (Fig. 3B and C) and presence (images are not shown) of the minimum concentration (as mentioned in Table 1) of positively charged surfactants are comparable; however, higher concentration leads to disaggregated particles, as evident from TEM images 3D and 3E. On the other hand, addition of the minimum concentration (as mentioned in Table 1) of neutral or negatively charged surfactants directly results in the disaggregated particles as shown in TEM images (Fig. 3F–H). The TEM images of samples after addition of surfactants show a decrease in polydispersity of the nanoparticles, which is correlated with the reduced LSPR band width and narrow DLS spectrum. Thus, one can tune the LSPR band by controlling the self-aggregation process using our easy and economic protocol.

To investigate the possible mechanism behind CDx-assisted formation and subsequent surfactant-induced dispersion of self-aggregates of Ag NPs, we performed zeta-potential (ζ) measurements. Fig. 4A shows the variation of ζ-potential with time after addition of 50 μM α- or β-CDx. The ζ-potential of synthesized Ag NP is +21.2 mV (±0.76), which decreases to +4.15 mV and +6.18 mV after addition of α- and β-CDx, respectively. Initially, the high positive surface charge (+21.2 mV) is due to the presence of positively charged CTAB on the particle surface, which is responsible for the inter-particle repulsion in order to avoid the aggregation. The decrease in surface charge of Ag NPs in the presence of CDx may be due to the following reasons: (a) attachment of CDx on the particle surface⁴¹ by removing surface attached stabilizer CTAB molecules; (b) insertion of CDx between nanoparticles through formation of inclusion complexes with stabilizer molecules⁴² and (c) detachment of CTAB from the NP surface through inclusion complexation with CDx⁴³ (Scheme 1). The surface charge will be negative for CDx stabilized Ag NPs at pH ≈ 8 (the pH of the initial Ag NP solution) due to the partial deprotonation of the hydroxyl groups of sugar-units at basic pH.^41,44 The almost unchanged positive ζ-potential of CDx-treated sample after 60 min (Fig. 4A) negates the possibility of attachment of CDx on the nanoparticle surface. Again replacement of CTAB on the surface with CDx signifies its higher binding affinity for NPs. However, the increase of positive ζ-potential through subsequent addition of CTAB to CDx-induced aggregated Ag NPs (Fig. 4B and C) should correspond to reattachment of CTAB, which is contradictory to the previously stated higher binding affinity of CDx for Ag NPs. Moreover, insertion of CDx between nanoparticles through formation of inclusion complexes with the surface stabilizer CTAB can lead to the decrease in ζ-potential through neutralization of the positive charge of CTAB with the partial negative charge of the hydroxyl groups of CDx at pH ≈ 8, which reveals that the nanoparticle surface remains intact after addition of CDx. Thus, subsequent addition of different surfactants to the Ag NP–CDx mixture will simply take away the inserted CDx through inclusion complexation, which results in the increase of positive ζ-potential for any type of surfactant due to the presence of unimpaired CTAB on the NP's surface. However, the unchanged ζ-potential after addition of a neutral surfactant, and some negative ζ-potential after addition of a negatively charged surfactant head-group to the Ag NP–CDx mixture (Fig. 4B and C) indicate that CTAB detaches from the NP-surface through formation of inclusion complexes with CDx. The detachment of CTAB from the surface is also manifested in the reshaping and resizing of the nanoparticles as shown in TEM images (Fig. 3). Hence, the decrease in surface charges, due to detachment of CTAB from the NPs surface via inclusion complexation with CDx, results in the self-aggregation. The greater extent of self-aggregation in the presence of α-CDx is correlated with the stronger inclusion complexation between CTAB and α-CDx than that of β-CDx.^43,45 In the aforementioned aggregation process, there still remains a small amount of surface charge on the nano-aggregates, which makes them stable in solution.⁴⁰ The re-attachment of added surfactants on the CDx-modified Ag NPs surface results in the breaking of self-aggregates, presumably due to the increase in ζ-potential for charged surfactants and the insertion of nanoparticles within the micelle for neutral surfactants (CMC of Tx-100 is 0.27 mM (ref. 46)). Thus, we have demonstrated an easy and inexpensive protocol for the generation of controlled self-aggregated Ag NPs without insertion of any self-assembling molecule.

Fig. 4 (A) Variation of zeta potential (ζ) of Ag NPs with time in the presence of 50 μM α- or β-CDx. Change in zeta potential (ζ) of (B) α- and (C) β-CDx pre-treated Ag NP samples in the presence of different concentrations of various types of surfactants.

Scheme 1 Schematic representation of the formation and subsequent dispersion of self-aggregates of Ag NPs.

4. Conclusions

In conclusion, we have demonstrated an easy, simple and economically inexpensive protocol for reversible LSPR tuning through formation and subsequent dispersion of self-aggregates of nanoparticles in a controlled way. Modulation of surface charge of Ag NPs by the addition of CDx and subsequent addition of different surfactants is responsible for the formation and dispersion of self-aggregates. The detachment of CTAB from the nanoparticle surface through inclusion complexation with CDx and subsequent reattachment of different surfactants on the nanoparticle surface is responsible for surface charge modulation of Ag NPs. This surface charge controllability of nanoparticles with various altered surface charges is significantly important in binding, disrupting, and penetrating many proteins, DNA, and cell membranes.^33,34,37 The simple and efficient method presented here enables large tunability of optical properties, which is significant for the application in developing highly sensitive chemical and biological sensors, optoelectronic devices and adaptive materials. Our approach is unique in the sense that we have been able to demonstrate shifting and reverse-shifting of the LSPR band via external additives, avoiding heavy metals unlike DNA-based systems as reported elsewhere.^29,30

6. Abbreviation

Ag NP	Silver nanoparticle
LSPR	Localized surface plasmon resonance
CDx	Cyclodextrin
CTAB	Cetyltrimethylammonium bromide
CpCl	Cetylpyridinium chloride
Tx-100	Triton X-100
SDS	Sodium dodecyl sulfate
SDBS	Sodium dodecylbenzene sulfonate
CMC	Critical micellar concentration

Acknowledgements

We thank DST SERB-India (Fund no. SB/S1/PC-041/2013) for financial support. NM thanks CSIR-India for his individual fellowship. We thank Prof. N. Sarkar for help in measuring DLS and ζ-potential in his lab-setup.

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