Nano graphene oxide-wrapped gold nanostars as ultrasensitive and stable SERS nanoprobes.

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Nano graphene oxide-wrapped gold nanostars as ultrasensitive and stable SERS nanoprobes Ghulam Jalania and Marta Cerrutia

Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/

We report a facile method to synthesize highly branched gold nanostars wrapped with nano graphene oxide (nGO) sheets with or without the addition of Raman dyes, as nanoprobes with high SERS activity. Both cysteamine and nGO are added to gold nanostars; the positively charged amino groups help self-assembly of nGO flakes around the nanostars. This increases the Raman signal of nGO by 5.3 folds compared to samples in which nGO is in contact with the nanostars but does not wrap them. We prepare also dye-based SERS nanoprobes by sandwiching typical Raman reporter such as Rhodamin B (RhB), Crystal Violet (CV) and Rhodamine 6G (R6G) between the nanostars and the nGO coating. The Raman signals of RhB is 5.2 times larger when sandwiched between nGO and nanostars than if the molecules are just adsorbed on the nanostar surface, and similar enhancements are observed for the other dyes. In addition to improving SERS efficiency, the wrapping greatly improves the stability of the dye-based nanoprobes, showing a reproducible Raman signal of RhB for over a week in simulated body fluids at 37° C. High SERS signal, facile fabrication method and excellent stability makes these nanoprobes highly promising for SERSbased biosensing and bioimaging applications. Surface enhanced Raman scattering (SERS) is an ultrasensitive technique for chemical sensing, biosensing, bioimaging, pollutant detection and cancer diagnosis.1-3 Typical SERS nanoprobes involve a Raman active reporter molecule (usually a dye) adsorbed on the surface of a plasmonic nanomaterial.4 The signal enhancement of the reporter molecules originates from two mechanisms, namely electromagnetic (EM) and chemical enhancement.5 EM enhancement happens when localised EM fields due to plasmons occurring at the surface of the

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nanomaterials interact with molecules present within 2-3 nm from the surface of the nanomaterials.6 Chemical enhancement is a less common mechanism believed to be due to charge transfer between nanomaterials and the adsorbed molecules.7 The combination of EM and CM can produce up to 109-1011 times enhancement of the signal.8 The intrinsic properties of nanomaterials largely affect the Raman signal enhancement. Ag and Au are the most widely used metals for SERS applications because their surface plasmons can be excited with most of the visible lasers used in Raman spectroscopy.2 Different geometries of Au and Ag have been used for SERS.9-14 Among them, Au nanostars (NSt)15, 16 shine due to their easy synthesis17, 18 and high SERS activity, related to the localized EM fields formed at the tips of the nanostar branches. Graphene oxide nanoparticles (nGO) are oxidised graphene sheets, usually in monolayer or few layered stacks, water dispersible because of the presence of oxygenated functionalities on their surface.19 nGO can be used as SERS enhancer, since the Raman signal of molecules adsorbed on nGO can be enhanced via CM.20, 21 In addition to this, the intrinsic Raman signal of nGO can be enhanced when decorated with Au and Ag NP; thus in this case nGO acts as a SERS reporter. The combination of both effects makes nGO a self-enhancing SERS tag. NP-nGO hybrids have been developed as label-free SERS platform for chemical sensing,22 biosensing23 and cellular imaging.24, 25 In all these reports nGO sheets act as supports for the metal NP. This configuration allows for a limited number of contact points between nGO and metal NP, and therefore few SERS hotspots. An efficient contact between nanomaterials and nGO is crucial to achieve a large enhancement of Raman signal.21 Recently, Ma et al. successfully wrapped nGO around Au NP to increase the number of hotspots between nGO and NP for intracellular SERS imaging.24 Other examples

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of nGO-wrapped nanostructures include Au nanorods for chemotherapy26 and mesoporous silica NP for drug delivery.27 A few reports show nGO sheets used in combination with dyetagged Au and Ag NP for SERS-based cellular imaging28 and cancer targeting.29, 30 In all these reports, again, nGO sheets act as a support of the dye-tagged NP. In this configuration only a few dye molecules are sandwiched between the NP and the nGO sheets, resulting in fewer hotspots and a less strong SERS effect. A common problem of SERS nanoprobes is the desorption of the Raman-active dye from the surface of the NP in physiological environments.31 To prevent dye desorption, the nanoprobes are often protected with an additional coating32 made of polymers,33 proteins34, 35 or silica.36, 37 In addition to preventing the desorption of Raman dyes, these coatings provide further sites to bind biological ligands for specific targeting.24, 38 Here we report a versatile and easy method to wrap nGO sheets around Au NP with different morphologies to produce SERS substrates. The design of SERS nanoprobes largely depends on the properties of the plasmonic nanomaterial and of the reporter molecules. We select Au NSt as plasmonic enhancers, and nGO both as a Raman reporter and a SERS enhancer, both in the absence (Au NSt@nGO) and in the presence of Raman dyes such as RhB (Au NSt@RhB@nGO), crystal violet (Au NSt@CV@nGO) and rhodamine 6G (Au NSt@R6G@nGO; see Table S1 for a list of all samples). Figure 1 shows a schematic of a single step procedure adapted in this work to wrap nGO around Au NSt. Au NSt are mixed with cysteamine and nGO to simultaneously achieve amino functionalization of Au NSt and wrapping with nGO. In addition to these NP, we prepared a series of control samples where nGO and unmodified Au NP were mixed (Au NSt-nGO and Au NSt@RhB-nGO, Au NSt@CV-nGO and Au NSt@R6G-nGO, see Table S1). We also tested Au nanospheres (Au NSp) to evaluate the applicability of the proposed nGO wrapping method to different sizes and shapes of Au NP.

Journal Name DOI: 10.1039/C4NR07473D While previous reports of nGO wrapping around Au NP did not involve any surface modification, here we modify the surface of Au NP with positively charged groups to promote the self-assembly of negatively charged nGO sheets39 around them. We select cysteamine as a surface modifier, since it contains a thiol group on one end, which can readily bind to gold by replacing the loosely bound sulfonate (on NSt) and citrate ions (on NSp) on the surface of NPs. Also, it has an amino group on the other end, which can provide a positive charge at pH lower than its pKa of 8.6.40 The initial pH of Au NSt is 7.4. When the Au NSt, the cysteamine and the nGO solutions are mixed, the thiol groups from cysteamine readily attach to the surface of the Au NSt while the amino groups extend outwards. The pH drops to 6.5±0.1, resulting in the protonation of the amino groups on the Au NSt. This in turn attracts the negatively charged nGO sheets,27 and causes them to wrap around the NP. To confirm the surface functionalization of Au NSt with cysteamine and wrapping with nGO, we perform X-ray Photoelectron Spectroscopy (XPS) on as-synthesized Au NSt and nGO-wrapped Au NSt. The atomic percentages of different elements at each stage are shown in Table 1. The presence of carbon (C) and oxygen (O) on assynthesized Au NSt can be ascribed to both contaminations and 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid) (HEPES) molecules adsorbed on the surface of Au NSt, while nitrogen (N) and sulfur (S) are due to HEPES. After functionalization with cysteamine and wrapping with nGO, the amounts of C and O increase while the amounts of Au, S and N decrease. This decrease must be related to the presence of a layer of nGO around Au NSt. More can be learned by looking at the high resolution spectra of C1s, N1s and S2p at each stage (Figure 2). The C1s spectrum of as-prepared Au NSt can be deconvoluted in three peaks centered at 285, 286.3 and 289 eV, which correspond to C–C, C–O and O–C=O bonds respectively41 (Figure 2A). The N1s spectrum shows a broad single peak at 399 eV, which can be attributed to the C–N–C bridges present in the aromatic ring of HEPES42 (Figure 2B).

Figure 1. Schematic illustration of the procedure used to prepare nGO wrapped Au NSt. Cysteamine and nGO are added to the Au NSt (route “A”); the presence of positively charged NH3+ groups on Au NSt help the electrostatic self-assembly of nGO around Au NSt. In a second set of samples, RhB, CV or R6G are added as Raman reporter molecules in addition to cysteamine and nGO (route “B”); the effect of nGO wrapping on the Raman signal intensity is investigated.

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Figure 2. High resolution XPS spectra of (A, D) C1s, (B, E) N1s and (C, F) S2p collected on (A-C) Au NSt and (D-F) Au NSt@nGO samples.

The S2p spectrum shows a doublet at 167.9 eV (S2p3/2) and 169.1 eV (S2p1/2), which corresponds to the –SO3 groups present in HEPES43 (Figure 2C). After functionalization with cysteamine and wrapping with nGO (Au NSt@nGO), the high-resolution spectra are very different. The C1s spectrum shows the typical peaks of nGO at 284.1, 284.8, 286.7 and 288.9 eV, which correspond to C=C, C–C, C–O/C–O–C and O–C=O respectively,41, 44, 45 (Figure 2D) thus confirming the presence of a layer of nGO. In the N1s spectrum, in addition to the peak at 399.1 eV due to C–N–C from HEPES, two new intense peaks appear, centered at 399.9 and 402.1 eV (Figure 2E). These can be related to C– NH2 from cysteamine and C–NH3+ from the protonated amino groups of cysteamine, respectively.42, 46 Also the S2p spectrum shows more peaks: in addition to the doublet already observed on as-prepared Au NSt, with two peaks at 168.1 (S2p3/2) and 169.3 (S2p1/2) eV, corresponding to the – SO3 groups from HEPES, there are two new doublets (Figure 2F). One of them, with peaks centered at 162.1 (S2p3/2) and 162.3 (S2p1/2) eV, can be related to the presence of Au–SH bonds, which confirm the binding of cysteamine to the Au NSt;43, 47 the other doublet has peaks centered at 164.1(S2p3/2) and 165.3 (S2p1/2) eV, and corresponds to the C–SH from free cysteamine molecules,48 most likely entrapped between nGO and Au NSt. The high resolution N1s and S2p spectra on Au NSp@nGO show peaks in the same positions as those discussed in relation to cysteamine for Au NSt@nGO (Figure S1). These results show the successful modification with both cysteamine and nGO on both Au NSt and Au NSp.

Table 1. Atomic % of different elements obtained from XPS survey scans of Au NSt and Au NSt@nGO samples. The errors represent standard deviation among three different points. Sample Au NSt

Au 11±1.5

C 51.5±0.8

O 21.5±1.6

N 9.2±0.7

S 6.8±1.3

Au NSt@nGO

4.1±0.1

49.1±3.6

43.8±4.5

3.7±0.6

3.1±0.3


We further confirm the wrapping of Au NSt with nGO using TEM. The images in figure 3 show that most of the Au NSt are composed of a core of 18.2±7.3nm diameter and 3 to 9 branches with lengths of 14.8±5.3nm (Figure 3A). After incubating NPs with cysteamine (12 µM) and nGO for 60 minutes, Au NSt show an nGO shell of 5.3±1.1nm (Figures 3B). The addition of Raman dyes (e.g. RhB) does not significantly change the thickness of the nGO shell (Figure 3C). Similar results are obtained on Au NSp (Figure S2). In the absence of cysteamine, nGO flakes do not wrap around the particles as well, and particles aggregate together with free nGO flakes are found both in case of Au NSt (Figure S3A) and Au NSp (Figure S3B). Overall these results thus show that by using cysteamine as a surface functionalization agent, it is possible to efficiently wrap nGO around Au NP of largely different diameters and shapes. This method works because cysteamine has a larger affinity to gold than both sulfonate ions, present on the surface of Au NSt synthesized in the presence of HEPES, and citrate ions, present on the surface of Au NSp.49 The importance of an efficient wrapping of nGO around Au NSt in order to obtain a good SERS effect is shown in Figures 3(D-G). Here we compare the Raman spectra of unmodified and cysteamine-modified Au NSt incubated with nGO, both in the presence and absence of Raman dyes. Spectra for the Au NSt in the presence of nGO alone are shown in Figure 3D. All spectra show the typical D band (1350 cm-1) and G band (1590cm-1) of graphene.50 The spectral intensity (compare for example the intensity of the D bands) increases in the order nGO< Au NSt-nGO < Au NSt@nGO (Figure 3D). The spectral intensity shows a similar trend in the presence of RhB (Figure 3E), CV (Figure 3F) and R6G (Figure 3G); in these spectra, the signals due to dyes completely overwhelm those due to graphene, so all the bands are related to molecular vibrations of Raman dyes. The signal enhancement while going from Au NSt@RhB to Au NSt@RhBnGO is 2.4 folds and from Au NSt@RhB-nGO to Au NSt@RhB@nGO is 5.2 folds.

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Figure 3. TEM images of (A) Au NSt, (B) Au NSt@nGO and (C) Au NSt@RhB@nGO. The samples were washed with DI water to remove any unreacted nGO and other reactants and redispersed in DI water prior to TEM imaging. (D) Raman spectra of Au NSt in the presence of nGO, prepared either with (black line) and without (red lines) cysteamine. Spectra show the distinct D and G peaks of nGO, also visible on the nGO spectrum (blue line). Raman spectra of Au NSt in the presence of nGO and (E) RhB, (F) CV and (G) R6G prepared either with (black lines) or without (red lines) cysteamine, and of just dye adsorbed on the Au NP (blue line).

These enhancements are 2.1 and 5.4 folds for CV-based nanoprobes, and 3.0 and 4.9 folds for R6G-based nanoprobes. Overall, these results imply that cysteamine improves the wrapping of nGO around the Au NP, and improves the SERS effect of all dyes sandwiched between the NP and nGO. We will use RhB as a model Raman dye for the rest of the experiments presented in this work. To wrap nGO around Au NP, both the number of positive surface charges on the NP and a homogenous NP dispersion are important. Thus, the cysteamine concentration greatly influences the wrapping process: too much cysteamine results in aggregation because the positive charges due to the protonated amino groups cancel out the negative charges that are responsible for the aqueous dispersion of the nanoprobes. On the other hand, low concentrations of cysteamine result in poor electrostatic interactions between Au NP and nGO, and therefore decrease the SERS enhancement. We find that 12 µM is the


optimal value, above which NP start to agglomerate, and below which lower SERS enhancement is obtained. This is shown in Figures 4 and 5. Figure 4A shows the intensity variation of the peak centered at 658 nm in the UV-Vis spectra of Au NSt@nGO samples prepared using cysteamine concentrations of 3, 6, 9, 12 and 15 µM. The representative UV-Vis spectra of Au NSt@nGO prepared with 12 µM and 15 µM cysteamine are plotted in figure S4A and figure S4B respectively. The intensity of this peak for Au NSt solutions incubated with 3, 6, 9 and 12 µM remains constant over a period of 30 minutes, thus showing that the particles are stable. However, the intensity of this peak starts decreasing quickly if the concentration of cysteamine is increased to 15 µM. This implies that Au NSt particles in contact with a 15 µM cysteamine solution quickly settle, and therefore 12 µM is the threshold concentration above which particle aggregation occurs.

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Figure 4. (A) Change over time of the intensity of the peak centered at 658 nm measured on UV-Vis spectra of Au NSt@nGO prepared with different cysteamine concentrations. (B) UV-Vis absorption spectra of Au NSt, nGO, Au NSt@nGO (cysteamine concentration:12 µM) and Au NSt@RhB@nGO (RhB concentration: 30nM, cysteamine concentration: 12µM)

The optical stability of particles prepared with 12 µM cysteamine concentration in the presence of both nGO and RhB is shown in Figure 4B where we plot the UV-Vis spectra of Au NSt@nGO and Au NSt@RhB@nGO samples. Spectra of Au NSt@nGO and Au NSt@RhB@nGO both show the characteristic peaks of Au NSt at 537 and 658 nm,18 thus confirming that the particles are stable and retain their optical properties upon wrapping. The spectra also confirm the presence of nGO around both types of particles, as can be seen by the change in baseline, which after the wrapping follows the same trend as in the nGO spectrum. The lower intensity of the 658 nm peak after the addition of nGO and RhB is due to the dilution of the Au NSt colloids. Also, the peak at 658 nm is blue shifted by ~2.5 nm in nGO-wrapped Au NSt samples (see vertical dotted lines): this is because after wrapping with nGO, the sharp tips of the Au NSt (responsible for the longitudinal band) appear more spherical to the incident light. A similar effect has been observed on silica-coated Au NSt in a previous report.51 Additionally, the spectrum of Au NSt@RhB@nGO shows a wide absorption band between 520 and 560 nm; this is due to the overlapping of the absorption peak of Au NSt centered at 537 nm and the absorption peak of RhB at 556 nm.52 To investigate the possible interactions between cysteamine and nGO and Au NSt and nGO, we record UV-Vis spectra of AuNSt-nGO

and cysteamine-nGO colloids (see Table S1 and Figure S5). The spectrum of cysteamine-nGO (Figure S5, green line) is quite similar to that observed for nGO (black line). The spectrum of Au NSt-nGO (red line) shows some increase in the baseline compared to Au NSt alone (blue line); however, no shift in the position of the 658 nm peak is observed. This probably indicates that while nGO does have some affinity towards Au NSt, the wrapping is not as efficient as in the presence of cysteamine, and the NSt tips remain as sharp as in the absence of nGO. This further confirms TEM results (Figures 3D-G and Figure S3), and explains why this configuration does not yield maximum SERS enhancement. While cysteamine concentrations higher than 12 µM cannot be used because they lead to particle aggregation, concentrations lower than this value should not be used because they lead to a lower SERS enhancement. Figure S6A shows the spectra of Au NSt@nGO measured with different cysteamine concentrations; all spectra show the same graphene bands observed in Figure 3. However, the spectral intensity greatly increases when cysteamine concentration increases. This is reported in Figure 5B (black line) by plotting the intensity of the D peak as a function of cysteamine. Since nGO concentration is constant (0.01 mg/ml) in all samples, the increase in Raman signal observed here must be attributed to a more efficient

Figure 5. (A) Raman intensity of the RhB band at 1651 cm-1 as a function of RhB concentration measured on Au NSt@RhB. (B) Raman intensity of the RhB band at 1651 cm-1 measured on Au NSt@nGO (black line) and Au NSt@RhB@nGO (red line) as a function of cysteamine concentration. (C) Raman intensity of the RhB signal at 1651 cm-1 measured on Au NSt@RhB@nGO as a function of incubation time.


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Figure 6. Raman intensity of the 1651 cm-1 band of RhB measured on spectra shown in figure S8 panels B (black line) and C (red line). Error bars represent the standard deviation among 3 different samples (N=3).

-wrapping of the nGO sheets at higher cysteamine concentrations, due to the presence of more positively charged amino groups on the Au NSt. A similar enhancement in SERS effect as a function of wrapping efficiency due to cysteamine concentration is observed in the presence of RhB, i.e. in the dye-based SERS nanoprobes. To test this, we first optimize the concentration of RhB that allows for maximum Au NSt coverage by measuring the Raman spectra of Au NSt@RhB at different RhB concentrations. The signal increases when the concentration of RhB is increased from 15 to 30 nM, but saturates above this value; this is shown in the form of spectra in Figure S7, and as a graph by plotting the intensity of the RhB peak centered at 1651 cm-1 as a function of RhB concentration in Figure 5A. These results indicate that 30 nM is the concentration that allows for maximum RhB coverage on Au NSt in this system. Using this RhB concentration, we measure the Raman spectra of Au NSt@RhB@nGO samples prepared with variable cysteamine concentration. The intensity of the 1651 cm-1 peak related to RhB increases in intensity when the concentration of cysteamine is increased from 3 to 12 µM (Figures S6B and 5B (red line)). This is similar to what was observed in the absence of RhB by measuring the D peak of nGO; again, this underlines the importance of good nGO wrapping, in this case to achieve a good contact between RhB and Au NSt. In both Au NSt@RhB@nGO and of Au NSt@nGO, the best wrapping is achieved at cysteamine concentrations as high as possible (12 µM), to maximize the SERS effect. To study the kinetics of nGO wrapping, we first incubate Au NSt with RhB for two hours under mild stirring, and then mix with both cysteamine and nGO and start recording Raman spectra (Figure S8). The Raman signals related to RhB start increasing after 10 minutes of incubation; the maximum rate of increase is achieved at around 35 minutes, and then the signals plateau at around 60 min (Figure 5C). The slow initial rate of increase of the Raman signal observed during the first 30 min is probably due the fact that nGO sheets are randomly oriented in solution, and require some time to align themselves around the irregularly shaped Au NSt. Once aligned in the right orientation, they quickly adhere to Au NSt and entrap RhB molecules between the surface of NP and nGO sheets (see schematic drawings shown in Figure 5C). This sandwiched structure results in a rapid increase of the Raman


signal after around 35 minutes of incubation. The rate of increase of the Raman signal slows down after 50 minutes of incubation, most likely because the Au NSt are completely covered with nGO sheets. The negative charge from nGO prevents further attachment of other nGO sheets, and the Raman signal saturates. These results indicate that the electrostatic assembly of nGO sheets around Au NSt in this system takes around one hour to complete. The wrapping of nGO around Au NSt does not only produce a high SERS signal but also increases the physical and chemical stability of the nanoprobes. Raman spectra collected on Au NSt@nGO (Figure S9A) and Au NSt@RhB@nGO (Figure S9B) incubated in PBS at 37°C for 7 days show almost no changes over time. Instead, the spectra recorded on Au NSt@RhB prepared by adsorbing RhB on the Au NSt without further wrapping with nGO greatly decrease in intensity over time (Figure S9C). A plot of the intensity of the RhB band at 1651cm-1 as a function of incubation time in PBS (Figure 6) clearly shows the difference in RhB signal stability in the presence (black line) or absence (red line) of nGO wrapping. To investigate the origin of this decrease in Raman signal, we record the UV-Vis spectra of Au NSt@RhB (Figure S10A) and Au NSt@RhB@nGO (Figure S10B) colloids over 8 days. The spectral intensity of both colloids does not change significantly over this time, which indicates that the colloids are stable and do not aggregate. Therefore, the main reason for the decrease in the intensity of the Raman spectra of Au NSt@RhB (Figure 6, red line) can be attributed to the desorption of dye molecules from the surface of the NP. These results suggest that nGO wrapping prevents RhB desorption, and makes the SERS nanoprobes highly stable at physiological conditions. Most likely, nGO prevents RhB desorption both because it provides a physical barrier, and because since it is negatively charged it can electrostatically interact with the positively charged RhB molecules.

Conclusions This is the first report that shows the synthesis of nGO wrapped Au NSt SERS nanoprobes. We achieved this by functionalizing the NSt surface with cysteamine molecules; positively charged amino groups from cysteamine electrostatically interact with negatively charged nGO sheets thus greatly enhancing the wrapping efficiency. This simple and versatile method can be applied to wrap nGO sheets around Au NP of different sizes and shapes, both in the presence and absence of Raman dyes such as RhB, CV and R6G. nGO wrapping increases the SERS effect and prevents RhB desorption in physiological conditions, thus greatly enhancing the physiological stability of the dye-based SERS probes. Additionally, nGO contains a large number of residual functional groups such as carboxyls, epoxides and hydroxyls, which can be used to conjugate a number of biological moieties for specific targeting. This will eliminate the need for extra surface modification steps, which are often required to attach biological ligands to SERS nanoprobes.

Acknowledgments This work was supported by the Canada Research Chair foundation, the Canada Foundation for Innovation, the Center for Self-Assembled Chemical Structures, Fonds de recherche du Québec–Nature et technologies, the Natural Sciences and Engineering Research Council of Canada, and the McGill Engineering Doctoral Award.

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a

Department of Mining and Materials Engineering, McGill University, Montreal, QC, H3A 0C5, Canada. ElectronicSupplementary Information (ESI) available: [Experimental details, supplementary TEM images, XPS

spectra, UV-Vis and Raman spectra of Au NSt are given in supplementary information]. See DOI: 10.1039/c000000

1. R. A. Tripp, R. A. Dluhy and Y. P. Zhao, Nano Today, 2008, 3, 31-37. 2. T. Vo-Dinh, H. N. Wang and J. Scaffidi, J. Biophotonics, 2010, 3, 89102. 3. A. S. D. S. Indrasekara, S. Meyers, S. Shubeita, L. C. Feldman, T. Gustafsson and L. Fabris, Nanoscale, 2014, 6, 8891-8899. 4. C. L. Zavaleta, B. R. Smith, I. Walton, W. Doering, G. Davis, B. Shojaei, M. J. Natan and S. S. Gambhir, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 13511-13516. 5. A. Campion and P. Kambhampati, Chem. Soc. Rev., 1998, 27, 241-250. 6. L. Jensen, C. M. Aikens and G. C. Schatz, Chem. Soc. Rev., 2008, 37, 1061-1073. 7. P. Kambhampati, C. M. Child, M. C. Foster and A. Campion, J. Chem. Phys., 1998, 108, 5013-5026. 8. E. C. Le Ru, P. G. Etchegoin and M. Meyer, J. Chem. Phys., 2006, 125, 204701-204714. 9. H. Ko and V. V. Tsukruk, Small, 2008, 4, 1980-1984. 10. H. Park, S. Lee, L. Chen, E. K. Lee, S. Y. Shin, Y. H. Lee, S. W. Son, C. H. Oh, J. M. Song, S. H. Kang and J. Choo, Phys. Chem. Chem. Phys., 2009, 11, 7444-7449. 11. J. Kneipp, H. Kneipp, W. L. Rice and K. Kneipp, Anal. Chem., 2005, 77, 2381-2385. 12. R. Bukasov, T. A. Ali, P. Nordlander and J. S. Shumaker-Parry, ACS Nano, 2010, 4, 6639-6650. 13. S. Lee, H. Chon, M. Lee, J. Choo, S. Y. Shin, Y. H. Lee, I. J. Rhyu, S. W. Son and C. H. Oh, Biosens. Bioelectron., 2009, 24, 2260-2263. 14. X. Su, J. Zhang, L. Sun, T. W. Koo, S. Chan, N. Sundararajan, M. Yamakawa and A. A. Berlin, Nano Lett., 2005, 5, 49-54. 15. C. Hrelescu, T. K. Sau, A. L. Rogach, F. Jackel, G. Laurent, L. Douillard and F. Charra, Nano Lett, 2011, 11, 402-407. 16. C. G. Khoury and T. Vo-Dinh, J. Phys. Chem. C., 2008, 112, 1884918859. 17. J. P. Xie, J. Y. Lee and D. I. C. Wang, Chem. Mater., 2007, 19, 28232830. 18. C. J. DeSantis, A. A. Peverly, D. G. Peters and S. E. Skrabalak, Nano Lett., 2011, 11, 2164-2168. 19. R. Raj, S. C. Maroo and E. N. Wang, Nano Lett., 2013, 13, 1509-1515. 20. W. Fan, Y. H. Lee, S. Pedireddy, Q. Zhang, T. X. Liu and X. Y. Ling, Nanoscale, 2014, 6, 4843-4851. 21. W. G. Xu, J. Q. Xiao, Y. F. Chen, Y. B. Chen, X. Ling and J. Zhang, Adv. Mater., 2013, 25, 928-933. 22. X. F. Ding, L. T. Kong, J. Wang, F. Fang, D. D. Li and J. H. Liu, ACS Appl. Mater. Interfaces, 2013, 5, 7072-7078. 23. Z. Fan, R. Kanchanapally and P. C. Ray, J. Phys. Chem. Lett., 2013, 4, 3813-3818. 24. Z. M. Liu, Z. Y. Guo, H. Q. Zhong, X. C. Qin, M. M. Wan and B. W. Yang, Phys. Chem. Chem. Phys., 2013, 15, 2961-2966. 25. Q. H. Liu, L. Wei, J. Y. Wang, F. Peng, D. Luo, R. L. Cui, Y. Niu, X. J. Qin, Y. Liu, H. Sun, J. Yang and Y. Li, Nanoscale, 2012, 4, 7084-7089. 26. C. Xu, D. Yang, L. Mei, Q. Li, H. Zhu and T. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 12911-12920. 27. S. Sreejith, X. Ma and Y. Zhao, J. Am. Chem. Soc., 2012, 134, 1734617349.

28. Z. Liu, C. Hu, S. Li, W. Zhang and Z. Guo, Anal. Chem., 2012, 84, 10338-10344. 29. C. Hu, Y. Liu, J. Qin, G. Nie, B. Lei, Y. Xiao, M. Zheng and J. Rong, ACS Appl. Mater. Interfaces, 2013, 5, 4760-4768. 30. Y. Wang, L. Polavarapu and L. M. Liz-Marzan, ACS Appl. Mater. Interfaces, 2014, 6, 21798–21805 31. P. J. Huang, L. K. Chau, T. S. Yang, L. L. Tay and T. T. Lin, Adv. Funct. Mater., 2009, 19, 242-248. 32. W. E. Doering, M. E. Piotti, M. J. Natan and R. G. Freeman, Adv. Mater., 2007, 19, 3100-3108. 33. G. Jalani, S. Lee, C. W. Jung, H. Jang, J. Choo and D. W. Lim, Analyst, 2013, 138, 4756-4759. 34. X. Su, J. Zhang, L. Sun, T. W. Koo, S. Chan, N. Sundararajan, M. Yamakawa and A. A. Berlin, Nano Lett., 2005, 5, 49-54. 35. L. Sun, K. B. Sung, C. Dentinger, B. Lutz, L. Nguyen, J. W. Zhang, H. Y. Qin, M. Yamakawa, M. Q. Cao, Y. Lu, A. J. Chmura, J. Zhu, X. Su, A. A. Berlin, S. Chan and B. Knudsen, Nano Lett., 2007, 7, 351-356. 36. J. Huang, K. H. Kim, N. Choi, H. Chon, S. Lee and J. Choo, Langmuir, 2011, 27, 10228-10233. 37. S. Lee, H. Chon, S. Y. Yoon, E. K. Lee, S. I. Chang, D. W. Lim and J. Choo, Nanoscale, 2012, 4, 124-129. 38. W. Ren, Y. Fang and E. Wang, ACS Nano, 2011, 5, 6425-6433. 39. Y. W. Zhu, S. Murali, W. W. Cai, X. S. Li, J. W. Suk, J. R. Potts and R. S. Ruoff, Adv. Mater., 2010, 22, 3906-3924. 40. S. P. Mezyk, J. Phys. Chem., 1995, 99, 13970-13975. 41. A. V. Shchukarev and D. V. Korolkov, Cent. Eur. J. Chem., 2004, 2, 347-362. 42. T. Heinrich, C. H. H. Traulsen, E. Darlatt, S. Richter, J. Poppenberg, N. L. Traulsen, I. Linder, A. Lippitz, P. M. Dietrich, B. Dib, W. E. S. Unger and C. A. Schalley, RSC Adv., 2014, 4, 17694-17702. 43. M. M. Nasef, H. Saidi, H. M. Nor and M. A. Yarmo, J. Appl. Polym. Sci., 2000, 76, 336-349. 44. G. Goncalves, M. Vila, I. Bdikin, A. de Andres, N. Emami, R. A. S. Ferreira, L. D. Carlos, J. Gracio and P. A. A. P. Marques, Sci. Rep. DOI: 10.1038/srep06735. 45. F. Pippig and A. Holländer, Appl. Surf. Sci., 2007, 253, 6817-6823. 46. G. M. Rignanese, A. Pasquarello, J. C. Charlier, X. Gonze and R. Car, Phys. Rev. Lett., 1997, 79, 5174-5177. 47. M. C. Bourg, A. Badia and R. B. Lennox, J. Phys. Chem. B., 2000, 104, 6562-6567. 48. M. M. Maye, J. Luo, Y. H. Lin, M. H. Engelhard, M. Hepel and C. J. Zhong, Langmuir, 2003, 19, 125-131. 49. J. Gao, X. Y. Huang, H. Liu, F. Zan and J. C. Ren, Langmuir, 2012, 28, 4464-4471. 50. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth and A. K. Geim, Phys. Rev. Lett., 2006, 97. 51. A. M. Fales, H. Yuan and T. Vo-Dinh, Langmuir, 2011, 27, 1218612190. 52. L. F. V. Ferreira, P. V. Cabral, P. Almeida, A. S. Oliveira, M. J. Reis and A. M. B. do Rego, Macromolecules, 1998, 31, 3936-3944.


J. Name., 2012, 00, 1-3 | 7



Notes and references

Nanoscale COMMUNICATION


Journal Name DOI: 10.1039/C4NR07473D

Table of contents entry only Gold nanoparticles (nanostars and nanospheres) are wrapped with graphene oxide sheets with or without the addition of a

positively charged amino groups and help self-assembly of nGO flakes. The resulting nanoprobes show high Raman signal and excellent stability under physiological conditions.



Raman dye, as ultrasensitive SERS nanoprobes. The surface of gold nanoparticles is modified with cysteamine to induce

8 | J. Name., 2012, 00, 1-3


Reduced graphene oxide-supported gold nanostars for improved SERS sensing and drug delivery.

Spectral Characterization and Intracellular Detection of Surface-Enhanced Raman Scattering (SERS)-Encoded Plasmonic Gold Nanostars.

L-cysteine-modified gold nanostars for SERS-based copper ions detection in aqueous media.

3D graphene nano-grid as a homogeneous protein distributor for ultrasensitive biosensors.

Highly stable SERS pH nanoprobes produced by co-solvent controlled AuNP aggregation.

Development of graphene oxide-wrapped gold nanorods as robust nanoplatform for ultrafast near-infrared SERS bioimaging.

Functionalisation of hollow gold nanospheres for use as stable, red-shifted SERS nanotags.

Multifunctional hybrid nanopatches of graphene oxide and gold nanostars for ultraefficient photothermal cancer therapy.

Ultrasensitive SERS detection of mercury based on the assembled gold nanochains.

On-line SERS detection of single bacterium using novel SERS nanoprobes and a microfluidic dielectrophoresis device.

Development of nanostars as a biocompatible tumor contrast agent: toward in vivo SERS imaging.

Nano-gold as artificial enzymes: hidden talents.

Nano-enabled SERS reporting photosensitizers.

Ultrasensitive opto-microfluidic immunosensor integrating gold nanoparticle-enhanced chemiluminescence and highly stable organic photodetector.

Functional nanoprobes for ultrasensitive detection of biomolecules: an update.

Plasmonic gold nanostars for multi-modality sensing and diagnostics.

DNA adduct identification using gold-aptamer nanoprobes.

Prussian blue-gold nanoparticles-ionic liquid functionalized reduced graphene oxide nanocomposite as label for ultrasensitive electrochemical immunoassay of alpha-fetoprotein.

Graphene oxide and shape-controlled silver nanoparticle hybrids for ultrasensitive single-particle surface-enhanced Raman scattering (SERS) sensing.

Stabilization and Encapsulation of Gold Nanostars Mediated by Dithiols.

Graphene-PDDA Nanohybrids for Bio-detection by SERS Nanotechnology.

An ultrasensitive supersandwich electrochemical DNA biosensor based on gold nanoparticles decorated reduced graphene oxide.

A SERS-active sensor based on heterogeneous gold nanostar core-silver nanoparticle satellite assemblies for ultrasensitive detection of aflatoxinB1.

Bio-Conjugated Gold Nanoparticle Based SERS Probe for Ultrasensitive Identification of Mosquito-Borne Viruses Using Raman Fingerprinting.