Introduction

Nanoparticles of various shapes, sizes, and compositions have been studied for a wide variety of applications, a non-comprehensive listing of which includes drug delivery (Yuan et al. 2012b; Nam et al. 2013), cancer diagnosis (Tamburro et al. 2011) and therapy (Cheng et al. 2011), contrast enhancers for imaging (Allijn et al. 2013), as well as surface-enhanced Raman spectroscopy (SERS) (Yuan et al. 2012a; Liu and Wu 2013; Jarvis and Goodacre 2004; Kastanos et al. 2009). Nanoparticles are relatively straightforward and inexpensive to synthesize through wet chemical preparation techniques. Well-known routes are available for almost any desired morphology. Of particular interest for SERS are branched gold nanoparticles, or nanostars, because of their peculiar plasmonic properties (Hao et al. 2007; Casu et al. 2012; Guerrero-Martinez et al. 2011; Rodriguez-Oliveros and Sanchez-Gil 2012; Kooij et al. 2012).

At present, many sophisticated synthetic routes ranging from seed-mediated (Kooij et al. 2012; Wu et al. 2009; Wheeler et al. 2012; Casu et al. 2012; Yuan et al. 2012c) to one-pot methods (Hrelescu et al. 2009; Trigari et al. 2011) are widely available for nanostars. The most common seed-mediated synthesis requires the use of cetyltrimethylammonium bromide (CTAB) (Rodriguez-Lorenzo et al. 2011; Esenturk and Walker 2009; Ahmed et al. 2010), polyvinylpyrrolidone (PVP), N,N-dimethylformamide (DMF) solution (Kumar et al. 2008; Khoury and Vo-Dinh 2008), and hydrazine (Jeong et al. 2009). Unfortunately, these surfactants are generally difficult to replace and challenging to functionalize for further applications (Trigari et al. 2011). Notable exceptions are the works of Casu et al. (2012) and Pallavicini et al. (2013), who have addressed this problem using zwitterionic lauryl sulfobetaine (LSB) surfactant, which gets easily removed compared to CTAB.

The straightforward synthesis of stable gold nanostars with no impeding outer layer is attractive from the viewpoint of applications. Yuan et al. (2012c) recently reported a protocol for producing surfactant-free nanostars with very little amount of citrate, a well-known biocompatible electrostatic stabilizer for gold nanoparticles. This protocol proves to be cost- and time-effective, in spite of being a seed-mediated route. However, in using this protocol, we have observed the occurrence of rapid aging (within 5 to 10 h) in all our suspensions of surfactant-free gold nanostars; hereafter, by aging we mean a morphological change of the nanoparticles that is reflected in the UV–visible absorbance spectrum. To our best knowledge, long-term stability and aging studies on surfactant-free nanostars have not been previously reported. A few papers on the stability of surfactant-stabilized nanoparticles exist. The stability studies performed by Trigari et al. (2011) were done solely on nanostars fabricated with CTAB capping and they found that even in the presence of surfactant, the nanostars can degrade after aging in suspension for some hours. Nanostars generally need to be coated to maintain stability. The use of thiolated compounds to maintain stability of nanostars is now becoming a standard. Examples of thiol-coated gold nanostars that gained excellent stability after surface coating with polyethyleneglycol-thiol (PEG-SH) were demonstrated in the works of Sironi et al. (2012) and Cavallaro et al. (2013). Another stability study was done by Gao et al. (2012), who used tandem coating of PEG-SH and MPA on spherical gold nanoparticles. Unlike these previous works, we focus on surfactant-free nanostars. We study the morphology and optical properties of surfactant-free nanostars upon aging at 4 °C, i.e., the common conditions of conservation for such kind of particles. We then propose to improve the existing protocol by forming a monolayer of MPA via Au–S bonding. One of the advantages of this approach is the solubility of MPA in water, which allows carrying out surface modification in an entirely aqueous environment.

Materials and methods

Sodium tetrachloroaurate (III) dehydrate (AuCl4Na·2H2O), sodium citrate tribasic dihydrate (Na3C6H5O7·2H2O), hydrochloric acid (HCl), silver nitrate (AgNO3), L-ascorbic acid (C6H8O6), and 3-mercaptopropionic acid (C3H6O2S) were purchased from Sigma-Aldrich and used without further purification. Milli-Q water was used for the preparation of all solutions.

According to the protocol proposed by Yuan et al. (2012c), we first synthesized spherical gold seeds. In a typical synthesis, aqueous solution of AuCl4Na·2H2O (100 ml, 1 mM) is brought to boiling point followed by rapid addition of sodium citrate (15 ml, 1 wt%). The solution was allowed to reflux for 15 min under vigorous stirring. After cooling down, the solution is kept at 4 °C. We then produced the nanostars by preparing a growth solution consisting of AuCl4Na·2H2O (10 ml, 0.25 mM), HCl (10 µl, 1 M), and 100 µl of gold seeds. Under moderate stirring, ascorbic acid (50 µl, 100 mM) and AgNO3 (100 µl, 1.5 mM) were simultaneously added. Instantly, a bluish-black color is achieved. We performed centrifugal washing at 3,500 rpm for 30 min. The extracted nanostars were redispersed in Milli-Q water and filtered with a 0.22-µm nitrocellulose membrane. The final product was kept at 4 °C. Surface modification of nanostars with MPA was done immediately after filtration by adding MPA (10 µl, 20 mM) to 10 ml of nanostars under vigorous stirring. The mixture was continuously stirred for 10 min at room temperature. No further centrifugal washing was done.

Absorption spectra in the wavelength range of 400–800 nm were measured using UV–vis spectrophotometer (Perkin-Elmer, Lambda 20 Bio). The morphological features of the particles were characterized by Transmission Electron Microscope (TEM, Philips EM 208). The nanostar colloids were monitored for several days after the synthesis. On the day of the measurement, suspensions were sonicated for 10 min before recording the UV–vis spectrum. An aliquot was further diluted to 10 % of the original stock concentration and 10 µl was drop cast onto a carbon TEM grid. The grid was dried at ambient temperature and TEM images were obtained on the same day of deposition. We used Image J software (ver. 1.46r) to examine the TEM images.

Results and discussion

Figure 1 shows a TEM image of the typical nanostars and one of the gold spherical particles used as seeds, together with their characteristic UV–vis extinction spectra. The gold spheres are highly monodisperse and have a mean diameter of 12.0 ± 0.4 nm. A suspension of such seeds exhibits a spectrum with a sharp absorption maximum at 520 nm. The nanostars, on the other hand, have a broader size distribution with an average diameter of 78 ± 13 nm (measured from tip to tip across the nanostar). TEM images reveal that a single nanostar can contain sharp as well as blunt tips. The UV–Vis spectrum obtained from the nanostars has a broader absorption peak at about 680 nm. We measured the absorption spectrum again after coating the nanostars with MPA. The primary absorption peak red-shifts by 10 nm, which is associated with the formation of a layer of MPA on the surface of the nanostars.

Fig. 1
figure 1

UV–vis absorption spectra of spherical Au seed (red), nanostars (blue), and MPA-coated nanostars (black). Insets TEM images of gold seeds and nanostars. The scale bars shown correspond to 100 nm. (Color figure online)

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We examined six batches of nanostars for this report. Three were surfactant-free, while the rest were coated with MPA. Figure 2 summarizes the changes in absorption maximum wavelength (λ abs) of different batches of nanostars. For surfactant-free nanostars denoted by SF1, SF2, and SF3, λ abs decreases with time indicating changes in the nanostar tip radius. Centrifugation and filtration do not effectively halt the aging process. λ abs continues to decrease in the following days of observation and the fall-off rate varies for different batches. SF1 has an initial absorbance at 689 nm and has the fastest decay rate, decreasing by as much as 67 nm within 9 days of aging. SF2 has an initial λ abs at 672 nm and it only decreased by 37 nm in a span of 9 days. The slowest decay rate was observed for SF3 with an initial λ abs at 693 nm and only decreased by 11 nm in 9 days. To show the long-term behavior of surfactant-free nanostars, we measured the UV–vis spectra of SF1 after 11 months. The suspension continues to decay, as evident from the further decrease of λ abs and change in color. SF1 turned to purple with λ abs of 585 nm. As we have noted, the different batches of surfactant-free nanostars have different aging kinetics. We are currently investigating whether the starting primary absorbance peak can possibly be the determinant of this decay rate. However, from the examples we presented in this paper, no conclusive correlation can be made. More tests should be done on this.

Fig. 2
figure 2

Shift of the wavelength at maximum absorption upon aging at 4 °C of surfactant-free (SF1, SF2, and SF3) and MPA-capped nanostars (M1, M2, and M3). The confidence on the wavelength is of the order of the dimensions of the data points

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To support our hypothesis that such changes in the extinction spectra can be explained in terms of morphological variations, we measured the radius of curvature of the nanostar tips. In Fig. 3, the radius of curvature of the nanostar tips after aging in solution at 4 °C is reported. Each column represents the average of about 200 tips measured from the TEM images and the bar represents the standard error. For the three batches of surfactant-free nanostars (SF1, SF2, and SF3), we report the average radius of curvature of the tip at day 0 (right after synthesis and filtration) and after one week of aging. For the three batches of MPA-capped nanostars (M1, M2, and M3), we report the radius at day 0 and after 30 days. The radius of the surfactant-free nanostars aged in solution for more than one week is consistently higher than the ones measured on the day of synthesis. On the other hand, no significant change is evident for the MPA nanostars even after 30 days. In Fig. 4, we show the extinction spectra of M1 for day 0, after 7 days, and after 6 months. The fact that the spectra remain unchanged confirms that there are no changes in the nanostar tips radius.

Fig. 3
figure 3

Changes in the mean radius of curvature (calculated over 200 nanostar tips) of freshly prepared (Day 0) and aged nanostars (after 7 days). Error bars represent one standard error

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Fig. 4
figure 4

Extinction spectra of MPA-coated nanostars, M1, recorded on day 0, after 7 days, and after 6 months

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An example of blunting of the tips in surfactant-free nanostars is presented in Fig. 5, which shows the nanostar SF1 at day 0, after 7 days, and after 11 months. At the bottom panel of Fig. 5, it is worth noting that the extinction spectrum after 11 months almost resembles that of a spherical particle with a larger diameter. This means that the contribution of the longitudinal mode to the plasmon resonance has decreased due to the loss of sharp protrusions. This is consistent with the observed decrease in the plasmon resonance wavelength of nanorods, which are comparable to the spikes of the nanostars, with decreasing aspect ratio (Vigderman et al. 2012; Huang et al. 2009). We are led to conclude that the gold atoms from the tips are migrating toward the core and filling the valleys, in a mechanism analogous to a typical sintering process whereby atoms move from convex to concave regions driven by the chemical potential difference associated to the respective curvatures (Kingery et al. 1976). We hypothesize that upon addition of MPA on the surface of the nanostars, the Au–S bond limits the mobility of Au atoms via surface diffusion and kinetically hinders the diffusion of the Au atoms from tips toward the core.

Fig. 5
figure 5

TEM image of nanostar SF1 at Day 0 (a), after 7 days (b), and after 11 months (c) (the scale bars shown represent 100 nm); (Bottom) extinction spectra of the same nanostars of a, b, and c

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Conclusion

Caution should be exercised when using surfactant-free suspensions of nanostars as they undergo rapid morphological evolution especially during the first few days after synthesis. The rate of decrease in the absorption maximum wavelength, as a consequence of the decrease in the aspect ratio of the nanostar spikes, slows down but does not plateau even after several months of aging. The surfactant-free nanostars, after almost one year of aging in suspension, become more spherical in shape with larger diameter compared to the seeds used in producing them. Aging is the result of migration of atoms from the convex tips to concave valleys toward the core of the nanostars. Such aging can be prevented by capping the nanostar surface with a thiolated compound such as MPA, without greatly affecting the optical property of the nanostars. From our study, we find that MPA-coated nanostars exhibit much longer stability than surfactant-free nanostars.