Open Access

Induced pH-dependent shift by local surface plasmon resonance in functionalized gold nanorods

Nanoscale Research Letters20138:103

DOI: 10.1186/1556-276X-8-103

Received: 27 September 2012

Accepted: 15 January 2013

Published: 22 February 2013

Abstract

Localized surface plasmon resonance (LSPR) spectroscopy of metallic nanoparticles is a powerful tool for chemical and biological sensing experiments. In this study, we observed LSPR shifts of 11-mercaptoundecanoic acid modified gold nanorods (GNR-MUA) for the pH range of 6.41 to 8.88. We proposed a mechanism involving changes of the dipole moment after protonation/deprotonation carboxylic groups of 11-mercaptoundecanoic acid (MUA) which plays an important role by modulating LSPR around the functionalized GNR. Such a stable and easily prepared GNR-MUA has potential to become one of the most efficient and promising pH nanosensors to study intra- or extra-cellular pH in a wide range of chemical or biological systems.

Keywords

Gold nanorods pH sensor LSPR 11-mercaptoundecanoic acid

Background

Gold nanoparticles have gained tremendous attention in recent years due to their attractive properties ranging from easy chemical synthesis, high biocompatibility, particular optical properties characterized by shape- and size-dependent localized surface plasmon resonance (LSPR), and locally enhanced-electromagnetic field. Among these noble metal plasmonic nanoparticles, gold nanorods (GNR) in particular, with its varied size, low reactivity, unique anisotropy shape, and optical properties, have been widely investigated by many research groups[13]. On the other hand, the LSPR frequency shifting has been widely used in chemical, gas[4] and bio-sensors[5], to examine the chirality of molecules[6] and be used as an electromagnetic energy transmitter[7] based on various types of pure-[8] or modified-metallic nanostructure array on glass substrate or nanoparticles in bulk solution[9]. In fact, developing of nanoparticle-based sensing materials is important and urgent for detection in special environment, for example, detection of single molecule analyte of internal cell[1012]. The free-label or monolayer/functionalized nanosensors have been achieved by fluorescence protein[13, 14], polymer[15, 16], quantum dots (QDs)[17], graphene oxide[18], and metal nanoparticles[19] through monitoring the variations in their fluorescence intensity or lifetime. However, the intrinsic drawbacks of fluorescence probe are photo-bleaching and blinking[20]. Furthermore, the cytotoxicity of the QDs makes them practically useless for in vivo biological application. Therefore, it is an urgent task to develop biocompatible and highly photostable nanoparticles for nanosensors, in particular, based on the extinction/scattering, and therefore, with non-blinking is highly preferential. Recently, Zijlstra et al. have demonstrated a label-free optical detection of single non-absorbing molecules by monitoring the plasmon resonance of nanorod via a sensitive photothermal spectra[21]. Generally speaking, optical sensors of metallic nanoparticles can be achieved by exploiting the sensitivity to local refractive index (n) of the surrounding medium (Δλmax ≈ Δn) or to the plasmon band shift that is caused by the proximity of nanoparticles[2124].

In this study, we investigate the pH-dependent local surface plasmon shift in a functionalized GNR. The gold nanorods modified by 11-mercaptoundecanoic acid (GNR-MUA) exhibit excellent stability and are easy to prepare, therefore can be the outstanding potential candidate for nanosensors. More importantly, it is based on the extinction spectrum (scattering) and thus non-blinking. We verified this optical signal originates neither from the aggregation of nanorods nor the variation of refractivity index through ion strength test and the pH titration procedure by comparing a modified pH-independent molecule (1-undecanethiol (UDT)) with MUA. We speculate that the dipole moment changes of MUA ligands on a rod surface play a very important role in this nanoparticle based-sensing system.

Methods

Materials

The materials used for the synthesis, hydrogen tetrachloroaurate (HAuCl4), sodium borohydrate (NaBH4), cetyltrimethylammonium bromide (CTAB), silver nitrate (AgNO3), l-ascorbic and ethanol, 95% (EtOH), nitric acid (HNO3), and sodium hydroxide (NaOH) 1 M were purchased from Sigma-Aldrich Corporation (St. Louis, MO, USA). 11-Mercaptopropionic acid (MUA) and UDT were of analytical grade and were obtained from Fluka (New South Wales, Australia). All standard chemical solutions or powders were protected from sunlight and kept at 25°C in a well-ventilated chemical storage cabinet and dry box. Stock solutions of sodium borohydride and l-ascorbic acid were freshly prepared for each new set of experiments.

Synthesis and sample fabrication

The GNRs (4.23 M) used in this study were synthesized by using the seed-mediated growth method in the presence of silver ions[25]. A 0.01 M MUA solution was prepared by mixing 0.04 g of MUA with 19.96 mL ethanol. A same concentration of UDT solution with MUA was prepared as mentioned above. The as-synthesized GNR was washed and centrifuged (6,000 rpm, 6 min) before 100 μL of MUA/UDT was added (remove excess cetyltrimethylammonium bromide (CTAB) surfactant). The LSPR peak of the samples was remained constant after 3 h of reaction time. Finally, the modified samples were washed before use to avoid unpredictable interferences from the free carboxylic groups of MUA in solutions.

Spectroscopic measurements

The morphology of each specimen was verified through TEM analysis (JEOL, JEM-1200EX 2, Akishima, Tokyo, Japan) operating at 80 kV. A double-beam UV–vis spectrophotometer (JASCO V-670, Easton, MD, USA) with a light path of 10 mm was used to measure the surface plasmon resonance of GNR. All measurements were performed at room temperature using 10-mm cuvettes. X-ray photoelectron spectroscopy (XPS) measurements were conducted using an ESCA Laboratory Thermo Scientific Theta Probe spectrometer (Waltham, MA, USA) with monochromatic Al Kα radiation (1,486.68 eV). C (1s) peak was used as an internal standard calibration peak at 284.6 eV.

Results and discussion

Figure 1a,b shows transmission electron microscopy (TEM) images and a particle size distribution of MUA which illustrates that no physical characteristic dissimilarity was found with as-synthesized GNR upon modification of GNR-MUA. The TEM image does not exhibit any corrosion, aggregation, or other defect (Figure 1a). The particle size analysis was carried out by counting about 100 particles for each specimen. It is estimated that the GNR has an average length of 53.93 ± 3.81 nm and diameter of 16.47 ± 1.76 nm, while the average length of as-synthesized GNR is 56.24 ± 3.47 nm and average diameter is 16.62 ± 1.60 nm (Figure 1b).
https://static-content.springer.com/image/art%3A10.1186%2F1556-276X-8-103/MediaObjects/11671_2012_Article_1345_Fig1_HTML.jpg
Figure 1

TEM, size distribution, UV-visible-IR extinction spectra, and functionalized GNR ligand. TEM images of GNR-MUA (a). Size distribution of GNR-MUA (b). Normalized UV-visible-IR extinction spectra of representative GNR-MUA in aqueous solutions (c). Schematic illustration of functionalized GNR ligand with CTAB, UDT, and MUA (d).

To ensure the integrity of each specimen and the formation of Au-S bond on GNR after MUA modification, we measured the characteristic extinction spectra, the XPS, and the zeta potential of as-synthesized GNR, GNR-MUA, and 1-undecanethiol modified gold nanorods (GNR-UDT) (Figure 1c). The LSPR spectral position is expected to be strongly affected by various factors such as the composition, formation and distribution of linkages, size, or shape of nanoparticles, as well as the refractive index of dielectric medium around them[26]. The as-synthesized GNR exhibited an absorption band centered at 850 nm. After the surface functionalization, a redshift of the extinction spectra was observed between GNR-MUA and GNR-UDT, at wavelengths 864 and 854 nm, respectively. The intensity of LSPR peak was found to be constant, but the FWHM of the peaks became broader for GNR-MUA and GNR-UDT as the gold-thiol bond formed[27].

XPS spectra measurement can confirm the formation of thiols bond to the Au surface. The XPS spectra shows that thiolates have S 2p binding energies of about 162.40 eV, whereas unbound thiols have those of 164 to 165 eV (Additional file1: Figure S1). This result is identical with the results of Zhao et al.[28]. Here, the C 1s peak at 284.88 eV was used as an internal standard calibration peak. The results also indicated that MUA was successfully bound to the surface of GNR. We further certified the degree of this replacement through zeta potential of GNR-MUA (Table 1). GNR displayed a very positive zeta potential (58.08 ± 0.6 eV) when CTAB dispersed on the metal surface. It has been noticed that there was an apparently decrease of zeta potential GNR-MUA (29.4 ± 0.6 eV) when surface GNR was modified with MUA. Besides, as the pH of GNR-MUA was adjusted from acid to base condition, the zeta potential becomes almost neutral. This result supports that CTAB coverage of GNR is partially displaced (Table 1).
Table 1

Zeta potentials and pH of GNR, GNR-MUA, and GNR-MUA after adding 30 μL NaOH

 

Zeta potential

pH

GNR

58.07 ± 0.55

3.92

GNR-MUA (0.03 M)

29.4 ± 0.6

7.49

GNR-MUA (+NaOH 30 μL)

8.69 ± 1.3

10.16

The face-selective modifications had been widely used in understanding and controlling the dynamics of self-assembled gold nanoparticles[29]. However, the mechanism of replacing CTAB is still an open question[30]. Here, the partially displaced surface can be explained by the following: First, according to the synthesis method of GNR by Sau et al., the GNR made in the presence of silver ions are single crystalline, with {111} facets on the long side of the rods[15]. On the other hand, it was reported that the surface energy of different facets generally increases in the order γ{111} < γ{100} < γ{110}[31]. Owing to the lowest surface energy among other facets and the particular structure of the {111} plane, thiol molecules like MUA and UDT preferentially bind onto it[32], which leads to the localization of monothiol MUA molecules prior at the long side of the rods (Figure 1d). Second, although the adsorption of a HS-containing aliphatic molecule onto the Au surface occurs very quickly, typically in few minutes at room temperature, Xia et al. believe that the presence of a compact bilayer of CTAB with high binding affinity to the surface of GNRs was responsible for the low coverage density of -S-PEG-NH2 chains on the CTAB-capped GNRs after ligand exchange[33].

To gain more insight about the relationship between LSPR and pH value, the plasmonic effect on the GNR-tethered MUA as a function of pH was studied using acid–base titration methods[34]. As Figure 1 shows, a 10.5 nm of LSPR shift of GNR-MUA (821.5 to 832 nm) was found after 30 μL of NaOH was added, similar to the result of Zijlstra et al., in which approximately 8-nm shift was detected with biotin receptors when the binding of single protein occurs[21]. At the same time, the plasmon peak exhibits redshift with increasing pH (pH 6.41 to 8.88) (Figure 2). It is noteworthy that this peak shift is not due to the aggregation of GNR because the self-assembly of GNR would led to a decrease in the absorption of the long wavelength band, accompanied by the formation of a redshifted absorption band[29, 35].
https://static-content.springer.com/image/art%3A10.1186%2F1556-276X-8-103/MediaObjects/11671_2012_Article_1345_Fig2_HTML.jpg
Figure 2

LSPR redshift of GNR-MUA after NaOH was added.

In addition, Figure 3 specifically summarizes the results of the absorption spectrum and the plasmon band intensity in a pH range of 3.8 to 8.88. It reveals a sigmoidal relation between LSPR shift and the volume of NaOH, when a 1- to 5-μL interval of NaOH was added. The sigmoidal curves of GNR-MUA (blue) before and after carboxylic acid deprotonation (red) seem to be right shifted compared with pure MUA (black) curve as a higher pKa value was found after MUA bound onto the metal surface[36]. Nevertheless, the position of LSPR band GNR-MUA added with different amounts of NaCl solutions (same concentration with NaOH) remain constant, which confirmed that the observed LSPR shift GNR-MUA was solely attributed to the pH changes instead of the combination effect from ionic strength (Additional file1: Figure S2). According to Sethi et al., a dramatic broadening and shift in LSPR that are caused by electrostatic aggregation of GNRs can occur in solution based simply upon the anions of the solvent used[37]. The addition of an analyte will induce the aggregation of nanoparticles, and the plasmon band will redshift due to coupling of surface plasmon.
https://static-content.springer.com/image/art%3A10.1186%2F1556-276X-8-103/MediaObjects/11671_2012_Article_1345_Fig3_HTML.jpg
Figure 3

LSPR shift of GNR-MUA versus NaOH volume.

Simultaneously, to verify that the LSPR shift of GNR-MUA was related to the charge on the surface of GNR, both LSPR of as-synthesized GNR and GNR-UDT were also estimated in the pH range of 3.8 to 8.88 (Figure 4). GNR-UDT is used here as a control which has the same chain length with GNR-MUA but uncharged terminal group. However, no LSPR shift was found. Moreover, the zeta potential of GNR-MUA also exhibits the critical role of functional group charge towards LSPR shift that decreases from 29.4 ± 0.6 mV to 8.69 ± 1.3 mV after adding 30 μL NaOH (Table 1). Furthermore, to verify the influence of free MUA in the solution towards the LSPR shift, we found that there was a consistence LSPR shift trend between washed and unwashed GNR-MUA samples. These results demonstrated that the observation of pH-dependent LSPR shift was apparently related to the changes in the charge of the carboxylic acid groups of MUA bond on GNR instead of free carboxylic groups of MUA (Additional file1: Figure S3).
https://static-content.springer.com/image/art%3A10.1186%2F1556-276X-8-103/MediaObjects/11671_2012_Article_1345_Fig4_HTML.jpg
Figure 4

Reversibility of LSPR shift from GNP, GNP-UDT, and GNP-MUA between pH 2.60 and 11.75.

Based on the above observation, subsequent experimental efforts have focused on the reversibility of the system. The titration procedure was repeated several times, going up and down on the pH scale. The LSPR of as-synthesized GNRs and GNR-UDT remains unchanged after the addition of 30 μL NaOH/HNO3 (Figure 4). This result is in good agreement with the result presented above that the LSPR of as-synthesized GNR and uncharged GNR-UDT was definitely not influenced by pH fluctuation. In comparison, the LSPR shift of GNR-MUA as a function of pH was found to be reversible between pH 11.75 and pH 2.60. Hence, these results indicate that the reversible change to the plasmon of these GNR tethered with MUA shows pH dependence, and this phenomenon demonstrates the utility of our pH nanosensor in a specific range of pH conditions.

The LSPR shift of GNR-MUA is 10.5 nm (821.5 to 832 nm) within the pH range of 6.41 to 8.88 (Figure 5). The S-shaped curve has a linear response range between pH 6.41 and 7.83. The slope of 5.11 indicated that there was a 5-nm shift of LSPR for each unit change of pH value. This pH-sensing range suggests potential application for pH determination in living-cell organelles such as endosomes and lysosomes, especially for the detection of specific tumor cells for which the cellular pH is within a range between 6.40 and 6.90[17].
https://static-content.springer.com/image/art%3A10.1186%2F1556-276X-8-103/MediaObjects/11671_2012_Article_1345_Fig5_HTML.jpg
Figure 5

LSPR shift of GNR-MUA ligands as a function of pH in solution.

It is well established that the peak wavelength, λmax, of the LSPR is dependent upon the size, shape, and distance between nanoparticles, as well as its dielectric properties and the changes in the effective refractive index (RI) of local surrounding environment including substrate, solvent, and adsorbates[38]. The dependence of LSPR or Fano resonance peak maximum[39] on RI which changes near the metal surface has been utilized in many plasmonic sensing applications. According to the modified equation of the LSPR wavelength shift Δλmax = m Δn(t/l) by Malinsky et al.[40], where m is the refractive index sensitivity, Δn represents the difference of local refractive index of protonated (nCOOH) and deprotonated ( n COO ) MUA, and t and l symbolize the thickness of the surrounding GNR after modification and decay length of the LSPR, individually in this experiment. From our refractive index measurements, there was no statistically significant difference between n COO and nCOOH. This suggests that there are very little changes in the local dielectric environment of protonated/deprotonated GNR-MUA nanoparticles. Therefore, our observation is not concordant with the equation mentioned above. However, the adsorption of thiol organic molecules can lead to the formation of microscopic surface dipoles that will modify the energy level alignment at the interface in both bulk and quantum dot semiconductors as observed in photovoltaic applications[41]. Here, the dipole moments calculated by DFT method for protonated and deprotonated MUA are 0.7 and 27.5 Debye, respectively (Figure 6). Thus, it is plausible that the redshift observed at higher pH is attributed to a relatively higher dipole moment of MUA as it is deprotonated. It is noteworthy that the formation of Au-thiol covalent bond shifts the LSPR to shorter wavelengths by approximately 10 nm, and it is due to the electron-donating nature of the sulfur headgroup in the molecule[42]. This means that the occurrence of the blueshift upon GNR happened while additional electrons were gained, while a redshift happened when part of the electrons were lost from the surface of GNR. The protonated/deprotonated MUA ligand that caused changes in the dipole moment of molecules may trigger various degrees of electron pulling force (the carboxyl groups of MUA are electron-withdrawing groups[43]). At a high pH, a larger electron-pulling force that restrains the electron-donating process of sulfur atom on MUA to the Au rod may cause the shift of LSPR to longer wavelengths, while a relative blueshift of LSPR occurs for GNR-MUA for a lower pH (Figure 6).
https://static-content.springer.com/image/art%3A10.1186%2F1556-276X-8-103/MediaObjects/11671_2012_Article_1345_Fig6_HTML.jpg
Figure 6

Schematic of electron-pulling force. On GNR-MUA to cause blue/red wavelength shift of LSPR at low and high pH.

Conclusions

In conclusion, a pH-dependent wavelength shift has been observed in GNR-MUA, which suggests that the charges formed on the surface of GNR after protonation/deprotonation of the carboxylic ligands of MUA play an important role by modulating LSPR phenomenon around the functionalized gold nanorods. Otherwise, -CH3-terminated ligand (CTAB or MUA) is independent of pH. The free MUA in the solution will not affect the LSPR shifting. In addition, we confirmed that the LSPR shifting is neither aggregation-induced optical signal nor the change of ionic strength. The LSPR shift of GNR is attributed to the dipole moment change after protonation/deprotonation of carboxylic groups of MUA. This GNR-MUA-based sensor can offer a 5-nm shift of LSPR for a unit change of pH value. Although the sensitivity of this GNR-MUA still has room for further improvement, such a stable and easily prepared GNR-MUA has potential to become efficient and promising pH nanosensors to study intra- or extra-cellular pH in a wide range of chemical or biological systems.

Declarations

Acknowledgments

The authors acknowledge Instrumental Center of National Taiwan University for the TEM. J. Tang thanks the support of the Academia Sinica and National Science Council of Taiwan under the program no. 99-2221-E-001-002-MY3 and 99-2113-M-001-023-MY3.

Authors’ Affiliations

(1)
Research Center for Applied Sciences, Academia Sinica
(2)
Institute of Cellular and Organismic Biology, Academia Sinica

References

  1. Zijlstra P, Orrit M: Single metal nanoparticles: optical detection, spectroscopy and applications. Rep Prog Phys 2011, 74: 106401. 10.1088/0034-4885/74/10/106401View Article
  2. Giljohann DA, Seferos DS, Daniel WL, Massich MD, Patel PC, Mirkin CA: Gold nanoparticles for biology and medicine. Angew Chem Int Ed 2010, 49: 3280–3294. 10.1002/anie.200904359View Article
  3. Mannelli I, Marco MP: Recent advances in analytical and bioanalysis applications of noble metal nanorods. Anal Bioanal Chem 2010, 398: 2451–2469. 10.1007/s00216-010-3937-8View Article
  4. Bingham JM, Anker JN, Kreno LE, Van Duyne RP: Gas sensing with high-resolution localized surface plasmon resonance spectroscopy. J Am Chem Soc 2010, 132: 17358–17359. 10.1021/ja1074272View Article
  5. Anker JN, Hall WP, Lyandres O, Shah NC, Zhao J, Van Duyne RP: Biosensing with plasmonic nanosensors. Nat Mater 2008, 7: 442–453. 10.1038/nmat2162View Article
  6. Hendry E, Carpy T, Johnston J, Popland M, Mikhaylovskiy RV, Lapthorn AJ, Kelly SM, Barron LD, Gadegaard N, Kadodwala M: Ultrasensitive detection and characterization of biomolecules using superchiral fields. Nat Nanotechnol 2010, 5: 783–787. 10.1038/nnano.2010.209View Article
  7. Waele RD, Koenderink AF, Polman A: Tunable nanoscale localization of energy on plasmon particle arrays. Nano Lett 2007, 7: 2004–2008. 10.1021/nl070807qView Article
  8. Beeram SR, Zamborini FP: Purification of gold nanoplates grown directly on surfaces for enhanced localized surface plasmon resonance biosensing. ACS Nano 2010, 4: 3633–3646. 10.1021/nn1007397View Article
  9. Mack NH, Wackerly JW, Malyarchuk V, Rogers JA, Moore JS, Nuzzo RG: Optical transduction of chemical forces. Nano Lett 2007, 7: 733–737. 10.1021/nl0629759View Article
  10. Jun YW, Sheikholeslamia S, Hostetter DR, Tajon C, Craik CS, Alivisatos AP: Continuous imaging of plasmon rulers in live cells reveals early-stage caspase-3 activation at the single-molecule level. Proc Natl Acad Sci 2009, 106: 17735–17740. 10.1073/pnas.0907367106View Article
  11. Srikun D, Albers AE, Chang CJ: A dendrimer-based platform for simultaneous dual fluorescence imaging of hydrogen peroxide and pH gradients produced in living cells. Chem Sci 2011, 2: 1156–1165. 10.1039/c1sc00064kView Article
  12. Pallaoro A, Braun GB, Reich NO, Moskovits M: Mapping local pH in live cells using encapsulated fluorescent SERS nanotags. Small 2010, 6: 618–622. 10.1002/smll.200901893View Article
  13. Tantama M, Hung YP, Yellen G: Imaging intracellular pH in live cells with a genetically encoded red fluorescent protein sensor. J Am Chem Soc 2011, 133: 10034–10037. 10.1021/ja202902dView Article
  14. Battisti A, Digman MA, Gratton E, Storti B, Beltram F, Bizzarri R: Intracellular pH measurements made simple by fluorescent protein probes and the phasor approach to fluorescence lifetime imaging. Chem Commun 2012, 48: 5127–5129. 10.1039/c2cc30373fView Article
  15. Sun H, Almdal K, Andresen TL: Expanding the dynamic measurement range for polymeric nanoparticle pH sensors. Chem Commun 2011, 47: 5268–5270. 10.1039/c1cc10439jView Article
  16. Zhou K, Liu H, Zhang S, Huang X, Wang Y, Huang G, Sumer BD, Gao J: Multicolored pH-tunable and activatable fluorescence nanoplatform responsive to physiologic pH stimuli. J Am Chem Soc 2012, 134: 7803–7811. 10.1021/ja300176wView Article
  17. Ruedas-Rama MJ, Orte A, Hall EA, Alvarez-Pez JM, Talavera EM: Quantum dot photoluminescence lifetime-based pH nanosensor. Chem Commun 2011, 47: 2898–2900. 10.1039/c0cc05252cView Article
  18. Chen JL, Yan XP: Ionic strength and pH reversible response of visible and near-infrared fluorescence of graphene oxide nanosheets for monitoring the extracellular pH. Chem Commun 2011, 47: 3135–3137. 10.1039/c0cc03999cView Article
  19. Lee CW, Takagi C, Truong TN, Chen YC, Ostafin A: Luminescent gold pH sensor based on nanoparticle-supported molecular brush. J Phys Chem C 2010, 114: 12459–12468. 10.1021/jp1033678View Article
  20. Nirmal M, Dabbousi BO, Bawendi MG, Macklin JJ, Trautman JK, Harris TD, Brus LE: Fluorescence intermittency in single cadmium selenide nanocrystals. Nature 1996, 383: 802–804. 10.1038/383802a0View Article
  21. Zijlstra P, Paulo PM, Orrit M: Optical detection of single non-absorbing molecules using the surface plasmon resonance of a gold nanorod. Nat Nanotechnol 2012. 10.1038/nnano.2012.51
  22. Nusz GJ, Marinakos SM, Curry AC, Dahlin A, Höök F, Wax A, Chilkoti A: Label-free plasmonic detection of biomolecular binding by a single gold nanorod. Anal Chem 2008, 80: 984–989. 10.1021/ac7017348View Article
  23. Strozyk MS, Chanana M, Pastoriza-Santos I, Pérez-Juste J, Liz-Marzán LM: Protein/polymer-based dual-responsive gold nanoparticles with pH-dependent thermal sensitivity. Adv Funct Mater 2012, 22: 1436–1444. 10.1002/adfm.201102471View Article
  24. Li DX, Zhang JF, Jang YH, JangYJ KDH, Kim JS: Plasmonic-coupling-based sensing by the assembly and disassembly of dipycolylamine-tagged gold nanoparticles induced by complexing with cations and anions. Small 2012, 8: 1442–1448. 10.1002/smll.201102335View Article
  25. Sau TK, Murphy CJ: Seeded high yield synthesis of short Au nanorods in aqueous solution. Langmuir 2004, 20: 6414–6420. 10.1021/la049463zView Article
  26. Sepúlveda B, Angelomé PC, Lechuga LM, Liz-Marzán LM: LSPR-based nanobiosensors. Nano Today 2009, 4: 244–251. 10.1016/j.nantod.2009.04.001View Article
  27. Linnert T, Mulvaney P, Henglein A: Surface chemistry of colloidal silver-surface-plasmon damping by chemisorbed I-, SH-, and C6H5S. J Phys Chem 1993, 97: 679–682. 10.1021/j100105a024View Article
  28. Zhao X, Cai Y, Wang T, Shi Y, Jiang G: Preparation of alkanethiolate-functionalized core/shell Fe3O4@Au nanoparticles and its interaction with several typical target molecules. Anal Chem 2008, 80: 9091–9096. 10.1021/ac801581mView Article
  29. Abbas A, Tian L, Kattumenu R, Halim A, Singamaneni S: Freezing the assembly process of gold nanocrystals. Chem Commun 2012, 48: 1677–1679. 10.1039/c2cc16356jView Article
  30. Murphy CJ, Thompson LB, Alkilany AM, Sisco PN, Boulos SP, Sivapalan ST, Yang JA, Chernak DJ, Huang J: The many faces of gold nanorods. J Phys Chem Lett 2010, 1: 2867–2875. 10.1021/jz100992xView Article
  31. Personick ML, Langille MR, Zhang J, Mirkin CA: Shape control of gold nanoparticles by silver underpotential deposition. Nano Lett 2011, 11: 3394–3398. 10.1021/nl201796sView Article
  32. Jathesh K, George Thomas K: Surface-enhanced Raman spectroscopy: investigations at the nanorod edges and dimer junctions. J Phys Chem Lett 2011, 2: 610–615. 10.1021/jz2000613View Article
  33. Xia X, Yang M, Wang Y, Zheng Y, Li Q, Chen J, Xia Y: Quantifying the coverage density of poly(ethylene glycol) chains on the surface of gold nanostructures. ACS Nano 2012, 6: 512–522. 10.1021/nn2038516View Article
  34. Wang D, Nap RJ, Lagzi I, Kowalczyk B, Han S, Grzybowski BA, Szleifer I: How and why nanoparticle’s curvature regulates the apparent pKa of the coating ligands. J Am Chem Soc 2011, 133: 2192–2197. 10.1021/ja108154aView Article
  35. Thomas KG, Barazzouk S, Ipe BI, Joseph STS, Kamat PV: Unidirectional plasmon coupling through longitudinal self-assembly of gold nanorods. J Phys Chem B 2004, 108: 13066–13068. 10.1021/jp049167vView Article
  36. Kalsin AM, Kowalczyk B, Smoukov SK, Klajn R, Grzybowski BA: Ionic-like behavior of oppositely charged nanoparticles. J Am Chem Soc 2006, 128: 15046–15047. 10.1021/ja0642966View Article
  37. Sethi M, Joung G, Knecht MR: Stability and electrostatic assembly of Au nanorods for use in biological assays. Langmuir 2009, 25(1):317–325. 10.1021/la802096vView Article
  38. Kreibig U, Vollmer M: Optical Properties of Metal Clusters. Heidelberg: Springer; 1995.View Article
  39. Lassiter JB, Sobhani H, Fan JA, Kundu J, Capasso F, Nordlander P, Halas NJ: Fano resonances in plasmonic nanoclusters: geometrical and chemical tunability. Nano Lett 2010, 10: 3184–3189. 10.1021/nl102108uView Article
  40. Malinsky MD, Kelly KL, Schatz GC, Van Duyne RP: Chain length dependence and sensing capabilities of the localized surface plasmon resonance of silver nanoparticles chemically modified with alkanethiol self-assembled monolayers. J Am Chem Soc 2001, 123: 1471–1482. 10.1021/ja003312aView Article
  41. Soreni-Harari M, Yaacobi-Gross N, Steiner D, Aharoni A, Banin U, Millo O, Tessler N: Tuning energetic levels in nanocrystal quantum dots through surface manipulations. Nano Lett 2008, 8: 678–684. 10.1021/nl0732171View Article
  42. McFarland AD, Van Duyne RP: Single silver nanoparticles as real-time optical sensors with zeptomole sensitivity. Nano Lett 2003, 3: 1057–1062. 10.1021/nl034372sView Article
  43. Wu Z, Jin R: On the ligand’s role in the fluorescence of gold nanoclusters. Nano Lett 2010, 10: 2568–2573. 10.1021/nl101225fView Article

Copyright

© Toh et al.; licensee Springer. 2013

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.