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Hierarchical Assembly of Nanostructures to Decouple Fluorescence and Photothermal Effect Loredana Latterini* and Luigi Tarpani Dipartimento di Chimica, Universita di Perugia and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati, Via Elce di Sotto 8, 06123 Perugia, Italy ABSTRACT: Hybrid nanoparticles are designed and produced by a two-step procedure with the aim to obtain optically controlled multifunctional nanomaterials. In particular, using a sol gel method in alcoholic/water media, silica particles doped with 9-aminoacridine molecules are prepared with a mean diameter of 31 nm, which preserves the fluorescent properties of the dye. In a second step, these nanoparticles are capped with a thin (7 nm-size) gold shell whose growth does not quench the emission of the dye as proven by steady-state and time-resolved fluorescence measurements. The careful choice of the organic dye and the control of the metal layer growth make possible to completely uncouple the fluorescence and the plasmon bands of gold. The selective photoexcitation of fluorescence or plasmon absorption, leading to heat release, has been tested on phospholipidic membranes loaded with the prepared hybrid particles. Under 400-nm irradiation fluorescence is activated, which is used to image the membranes; upon 650-nm irradiation only the gold layer absorbs and efficiently converts light into heat leading to a temperature increase of about 10 °C in the surrounding medium which is responsible for the alteration of the membrane architecture.
1. INTRODUCTION Fluorescent hybrid nanoparticles are widely investigated as probes, sensors, photon sources, and light collectors.1,2 A central focus in nanomedicine research is the development of sub-100-nm structures as contrast agents, delivery vehicles, or therapeutics for improving the diagnosis and cancer treatments.3 The recent technical and instrumental developments allow treatments and manipulations on the nanometer scale becoming easily feasible thus the development of materials and methods to monitor the operations in situ are necessary. This aspect is important in material science, and it is fundamental for the establishment of nanomedicine. The use of fluorescence signals to monitor the nanotreatments is making the detection much easier compared to other signals and allow to reach a very sensitive response.4 In recent years, nanotechnology methods were developed, and nanomaterials can now be processed to obtain multicomponent nanomaterials with different functionalities.5,6 In this context, silica nanoparticles (Si-NPs) represent a solid template to be used for multifunctional materials since they can be prepared controlling the dimension and the surface properties;7 indeed, Si-NPs can entrap organic fluorescent molecules and tune the guest/guest and guest/matrix interactions2,8 to reduce unwanted energy transfer processes and prevent aggregate (or complex) formation thus leading to an improvement in the dye photostability and emission intensity.2,9 Furthermore, the ease to process the silica surface makes straightforward the insertion of different functionalities into a single Si-NP,10 with the aim to improve its operational functions. Silica nanoparticles can be used as dielectric cores to grow a gold shell that absorbs in the red/NIR region and efficiently converts the absorbed photons into heat r 2011 American Chemical Society
which can be dispersed in the surrounding medium.11 Gold nanostructures are particularly interesting; their optical and photophysical properties together with their stability and biocompatibility make them attractive in material science and for biomedical applications.12 When gold forms a thin layer on a dielectric material (such as silica) a coupling between the internal and external surface plasmons takes place and interactions with the dielectric interface occur making the surface plasmon resonance (SPR) band tunable in the near-infrared region (NIR, 800 1200 nm),11,12 where both blood and soft tissues are highly transparent and ensure a deep penetration of the light. Gold nanoshells are able to efficiently convert the absorbed light into heat and generate a temperature increase in the surrounding media.11 When the shell is complete (strong coupling) the shift of the SPR band is remarkable and the extinction contribution in the 400 550 nm region is much lower.12 This window can be used to independently activate and monitor the fluorescence of a selected dye introduced as dopant in the dielectric core. It is well know in the literature that interactions between dye molecules and metal nanostructures induce a fluorescence enhancement13 or emission quenching,14 depending on the fluorophore metal distance and on the coupling among the electronic levels of the dye and the optical properties of the metal nanostructures.13,15 Although fluorescence enhancement through metal nanoantennas can improve the sensitivity and the resolution of the fluorescence signals,16 this gain is achieved by designing nanomaterials in which Received: August 23, 2011 Revised: September 21, 2011 Published: September 26, 2011 21098
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The Journal of Physical Chemistry C the electronic levels of the dye and the optical properties of the metal structures are strongly coupled.15 Instead, particular attention has to be paid to the design and preparation of nanomaterials with uncoupled optical properties, namely fluorescence and SPR, in order to be able to activate them independently. To achieve these properties, multifunction hybrid nanoparticles can be used, and they have to be carefully designed and prepared. In particular, the metal layer has to be grown as thin as possible and up to reach a complete shell to maximize the SPR shift and create a range in the visible spectrum with a negligible extinction, where the fluorescence coming from the doped nucleus can be measured. Furthermore the building up of a complete gold shell excludes the risk of contact between the photoreactive parts of the particle (dye and silica) and the environments where the material is located, thus eliminating possible phototoxic effects. The organic dye to be trapped in the silica nucleus has to be selected to have high fluorescence efficiency and to have the absorption/emission spectra in the 380 500 nm region where the gold shell has a minimum contribution to extinction. 9-Aminoacridine (AA) is a fluorescent dye that absorbs ad emits in 400 500 nm region; it has a high fluorescence efficiency,16 although it strongly depends on the environment.18 The entrapment of the dye in a silica matrix can increase the dye photostability and can ensure the control of its distance from the metal layer to avoid quenching effects. In the present work, the preparation of engineered nanoparticles based on a two-step procedure, which has been optimized in order to preserve the fluorescence properties of AA, is presented. The hybrid colloids have been characterized by steady-state and time-resolved fluorescence techniques and by transmission electron microscopy (TEM) imaging. The selectivity of the optical activation of fluorescence and theranostic action has subsequently been tested on phospholipidic membranes.
2. EXPERIMENTAL SECTION Nanoparticles Preparation. The silica nanoparticles were prepared following literature procedures.19 Briefly, AA (5.0 10 7 M) and triethoxysilane (TES, 7.0 10 3 M) were codissolved in 50 mL of ethanol. To start the hydrolysis of TES, 3 mL of ammonium hydroxide were added dropwise under vigorous stirring. After 1 h, tetraethylortosilicate (TEOS, 0.15 M) was added to the reaction mixture and the solution was kept under stirring for 23 h. The particle surfaces were functionalized with amino groups by adding 100 μL of 3-triethoxysilylpropylamine. These amino-functionalized silica nanoparticles were then suspended in water and used as nuclei to grow the gold shell.20 Colloidal gold seeds were previously prepared in Milli-Q water by reduction of gold(III) chloride hydrate (6.3 10 5 M) using sodium citrate dihydrate (4.8 10 4 M) at 80 °C. Two mL of aminated silica bead solution was added to 20 mL of gold colloid solution with continuous stirring for 2 h to conjugate the gold nanoparticles onto silica beads. The gold nanoshells were grown on the seed decorated particles at 60 °C by adding dropwise 4.8 10 4 M of HAuCl4 and 6.3 10 5 M of sodium citrate, as already descrided.11a Vesicle Preparation. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles were prepared using the lipid film hydration method.21 Briefly 13.6 mg of DMPC were dissolved in CH3Cl in a 50-mL round-bottom flask, and a lipid film was prepared by evaporation of the solvent under a gentle N2 stream.
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The obtained film was overnight stored in a desiccator under reduced pressure and it was used to form the vesicles by hydration with 20 mL of DI water at 40 °C. The final concentration of lipid was 1.0 mM. The prepared DMPC vesicles were used for the experiments without any further purification step. Irradiation Procedure. The sample irradiation experiments were performed by use of a 20 W Halogen lamp equipped with band-pass filters to select the irradiation wavelength range (λirr = 650 nm, bandwidth 40 nm) and neutral density filters to control the irradiation fluence. In the comparative light dose experiments the three samples (Au@ SiO2-AA with complete shells, aminated SiO2-AA conjugated to gold seeds and SiO2-AA, respectively) were irradiated in parallel by splitting the light source in three different lines to avoid the effects of possible source fluctuations. Instrumentation. A Philips transmission electron microscope (model 208, operating at 80 kV of beam acceleration) was used to analyze the nanoparticle size distribution. The nanoparticles suspensions were deposited in a 400 mesh copper-coated with Formvar support grid and were left overnight in a desiccator to allow the solvent to evaporate. The size distribution histograms for the samples were obtained by analyzing 150 200 nanoparticles in each sample. An atomic force microscope (Solver-Pro P47H, NT-MDT) was used to record topographic images of the prepared nanoparticles once a drop of the water suspension was placed on mica and spin-coated to spread the particles during solvent removal. The measurements were carried out in semicontact conditions by use of 190 325 kHz cantilever.2 A Perkin-Elmer Lambda 5 spectrophotometer was used to monitor the optical properties of shells in the 350 1100 nm range. Corrected fluorescence emission and excitation spectra were acquired with a Fluorolog-2 (Spex, F112AI) fluorimeter. The fluorescence decay times τF, were measured by single photon counting method using an Edinburgh Instrument 199S setup. A 370-nm nanoLED with a 1.3 ns pulse duration was used as excitation source and the signal was acquired by a Hamamatsu R7400U-03 detector. For the fluorescence measurements, the concentration of the dye in the samples (embedded in the silica matrix) is estimated to be in the range 10 8 to 10 9 M. The fluorescence images were recorded through the laser scanning confocal microscope (Nikon, PCM2000) previously described,22 using a diode laser (λexc = 400 nm) as light source. The images were obtained with a 60, 1.4 N.A. oil immersion objective (512 512 pixels).
3. RESULTS AND DISCUSSION 3.1. Nanoparticle Preparation. The hybrid nanoparticles were prepared in a two-step procedure and the synthesis was optimized to preserve the fluorescence (see below). Silica nanoparticles loaded with AA (SiO2-AA) were synthesized by a sol gel method, by codissolving 9-AA and TES in ethanol in optimized concentrations and let them to react in basic conditions for 1 h. Afterward, the silica nanoparticles were further grown by addition of TEOS, and the reaction mixture was kept under vigorous stirring for additional 23 h. The use of the more lipophilic TES in the first step of the sol gel process is used to improve the adsorption of the dye inside the silica matrix.19 The obtained particles (in the same suspension) were then functionalized by condensation of APTES. At this stage the nanoparticles were centrifuged and washed to eliminate the eventual unreacted material and then resuspended in water. 21099
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The Journal of Physical Chemistry C The functionalized SiO2-AA nanoparticles were used as nuclei to build up gold nanoshell using a seeded growth strategy20 and taking care that the emission of the dye would not be quenched (see below). Previously prepared gold seeds were then conjugated to the aminated silica particles. To have a uniform shell growth a homogeneous seed conjugation is desirable. A more homogeneous coverage was obtained by adding the functionalized silica nuclei to a suspension of small (few nanometer in diameter, see below) gold seeds under vigorous stirring. This approach prevented seed accumulation and/or clustering on the silica. The obtained suspension was used to grow the gold layer by controlled reduction of Au(III) by citrate ions. The reactant amounts were optimized to obtain a complete shell as thin as possible, trying to favor a diffusion controlled deposition of the reduced gold atoms. The growth of the shell was checked by VIS-NIR spectroscopy and TEM imaging. The optical properties of the gold nanoshell
Figure 1. Extinction spectra of gold seeds (blue), aminated SiO2-AA with gold seeds (black), SiO2-AA during gold shell growth (green), and complete shell (red).
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suspensions were monitored in the 350 1100 nm (Figure 1). Upon growth of the metal layer on the silica surface, the SPR band (originally due to the gold seeds) shifted to the red and became broader to reach a structureless band in the 500 1100 nm region; in agreement with literature data reported for gold nanoshells,13 the broad and structureless band with maximum at 730 nm was assigned to complete gold shells. 3.2. Nanoparticle Characterization. TEM images (Figure 2) and AFM data reveal that the shape of AA loaded nanoparticles is spherical. The diameter distribution of the particles (Figure 3) has been built up from TEM images and analyzed by a Gaussian function. The distribution is centered at 31 nm and presents a fwhm of 4 nm indicating that the colloidal material is nicely monodispersed. Figure 2 shows representative images taken during the metal layer growth and at the end of the growth process. Figure 2b shows the functionalized silica particles conjugated with the gold seeds (darker dots), in which a quite homogeneous coverage of the silica surface with small gold seeds (3 5 nm diameter) can be observed. During the further gold layer growth, TEM images showed areas with different contrast (Figure 2c) due to the growing gold island (darker) over the silica particles (lighter).9 This image indicated that the shell formation occurred through the growth of metal seeds, thus accounting for the importance to start with a homogeneous silica coverage to reach an uniform shell. Indeed, using APTES-functionalized SiO2 nanoparticles not conjugated with the Au seeds as starting material for the preparation of the nanoshells brought to the formation of big clusters of particles covered with an inhomogeneous layer of gold. At the end of shell growth (Figure 2d), the nanoparticles appeared completely coated with a metal shell, as indicated by the homogeneous contrast distribution and the absence of seeds accumulation, which is normally observed when the shell growth is not complete.8 The metal layer dimension was determined by comparison between the size distribution before and after gold
Figure 2. TEM images of SiO2-AA (a), aminated SiO2-AA with gold seeds (b), during the shell growth (c), and complete gold shell (Au@ SiO2-AA, d); scale bars correspond to 50 nm. 21100
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Figure 3. Size distribution histograms obtained from TEM images of SiO2-AA (left panel), and complete gold shell (right panel).
layer treatment. In particular, the diameter distribution of the gold nanoshells, determined from the analysis of TEM images, is well described by two Gaussian distributions with peaks at 45 ( 3 and 53 ( 3 nm (Figure 3b) indicating the presence of two subpopulations of gold nanoshells. The lower diameter population is assigned to those nanoshells for which a complete gold shell around the AA-SiO2 nucleus has grown, in agreement with spectrophotometric data; the comparison of the mean diameter of the nanoparticles before and after the deposition of the gold layer indicates that in the present conditions a shell with 7 nm thickness was grown. The larger diameter (53 ( 3 nm) observed in the second population with a smaller weight in the distribution is likely due to the deposition of a second partial gold layer in some particles, as previously observed.11a It has to be noted that a small deviation from perfectly round shapes can be observed in some particles, as compared to the starting AA-SiO2 structures, suggesting that in the present conditions a preferential axial growth of the shell occurred. This can be due to a stabilizer effect although a contribution from surface controlled growth cannot be excluded.23 However no further efforts were done to prevent deviation from spherical shape since in agreement with literature data12 the elongated gold structures display a more intense red absorption due to the plasmon oscillation along the elongated axis of the particles. This plasmon effects will enhance the red absorption of the hybrid particles. 3.3. Fluorescence Properties of the Hybrid Nanoparticles. The fluorescence properties of the chromophore are not strongly affected by entrapment in the silica matrix. The emission and excitation spectra of dye loaded nanoparticles are shown in Figure 4 together with the spectra recorded from the dye in ethanol. The comparison of the emission spectra of AA in alcoholic solution and loaded in the silica nanoparticles shows that the spectral shape is similar suggesting that the organic molecule is mainly present in a monomeric form (since the aggregate species present a red-shifted and structureless emission spectra).24 Different from what observed in homogeneous solutions where AA has a monoexponential fluorescence decay (Table 1), the fluorescence of SiO2-AA particles revealed complex decays which could be satisfactorily fitted by biexponential functions. This type of nonexponential decay is correctly explained in terms of decay time distributions. Bimodal decay time distribution models were already proposed for fluorophore adsorbed on silica surfaces;2,18 in the present system this behavior is assigned to a distribution of decay times reflecting the fluorophore environments in the silica particles, since aggregate formation has been already excluded (see above). The best fitting of SiO2-AA fluorescence decay gave 8.6
Figure 4. Fluorescence spectra of (a) 9-AA in water (black line), SiO2-AA in water (blue line), and (b) Au@SiO2AA in water (black line) together with the extinction spectra of gold shell in the same spectral region.
Table 1. Fluorescence Decay Parameters of 9-AA in Different Environments τF1 (ns)
A1 (%)
AA/EtOH
14.0
100
AA/H2O
15.0
100
8.6 4.4
23 30
sample
SiO2-AA/H2O Au@SiO2-AA/H2O
τF2 (ns)
A2 (%)
20.5 24.6
77 70
and 20.5 ns decay time values (Table 1). It is known that acridine dyes are fluorescence probes affected by environment, pH, and solubilization site. The comparison of these results with literature data18 suggests the assignment of the longer component to the protonated dye molecules (formed upon interaction with residual SiOH groups) and the short living component to the unprotonated fraction. The fluorescent silica nanoparticles were then functionalized with amino groups to be used as cores to build up the metal shells. The fluorescence spectra of the dye were not altered by amine functionalization. The metal layer was deposited in controlled conditions to prevent drastic modifications of the fluorescence properties of the doped silica nuclei. Indeed the fluorescence of the chromophore could be still easily detected by direct and selective excitation of the dye in the hybrid material in the 370 420 nm range (where the excitation spectrum of the metal layer has a 21101
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Figure 5. Fluorescence images (λexc= 400 nm) of phospholipidic membranes loaded with SiO2-AA (a), Au@SiO2-AA (b,c).
lower contribution), although a 10 nm blue shift of the whole spectrum (and excitation spectrum as well) was observed (Figure 4). It has been previously reported that the electronic transitions of 9-AA derivatives are affected by medium polarity-polarizability25 as well as interactions with metal nanostructures26 and the spectral changes assigned to the presence of different prototropic species, changes in the nature of the lowest singlet state or to formation of dye-metal complexes. Fluorescence decay time measurements indicate that a consistent fraction of the fluorescence is stabilized by the metal layer growth (Table 1) and only the short living population undergoes a further fluorescence quenching. All these experimental observations indicate that the metal layer is able to modify the properties of the dye environment more than to establish a direct interaction with 9-AA.. Thus a hypothesis can be drawn on the dye localization in the nanostructures. In particular the short living population can probably be the fraction of the dye molecules more exposed to the silica surface and hence in closer proximity to the aminated surface and then to the metal layer. Taking into account the spectral behavior and the decay time distributions of the dye embedded in the silica particles, the data indicate that the fluorescence quantum yields of the fluorophore with and without the metal layer are of the same order of magnitude. However, further experiments are currently in progress in order to quantitatively understand the effects of the metal layer on the emission properties of the entrapped dye. 3.4. Fluorescence Imaging and Photothermal Effect of the Hybrid Nanoparticle on Lipid Bilayers. The dimensions and the chemical composition of the prepared fluorescent nanoshells allow to consider the material easily biocompatible for the uptake without further surface processing; thus tests were carried out with phospholipidic membranes to check the labeling capacity of the nanoparticles and the response to red irradiation. Cell membranes are constituted by a large variety of components, but the main unit acting as physical barrier and regulating the movement of the molecules in and out the cell is the phospolipidic bilayer. Thus, liposomes provide a good simplified model to investigate the interaction between lipid bilayers and a series of molecules such as DNA, surfactants, drugs, or nanoparticles.27 In recent studies, the ability of nanostructured materials to target the liposome has been evaluated to correlate these processes with nanoparticle penetration inside the cells and their cytotoxicity.28 DMPC vesicles were loaded with SiO2-AA particles or with gold-capped nanoparticles. In both cases, upon direct excitation of the embedded dye (λexc = 400 nm), the emission properties of fluorescent nuclei allowed to image phospholipidic membranes (Figure 5). In particular, the images revealed that the multicomponent nanoshells tend to accumulate at the interface between the water media and the hydrophobic layer of the vesicles where clustering phenomena cannot be excluded; the comparison
with the images recorded on the vesicles using the starting SiO2-AA colloids suggests that the multicomponent nanoshells have a lower degree of permeability through the membrane likely due to the hydrophilic nature of the citrate ions present on the surface of the nanoparticles to act as stabilizer of the metal shell. The emission intensity of the fluorescent gold-capped nanoparticles is sufficient to record optical images; even under prolonged irradiation at 400 nm no bleaching and sample degradation/ alteration was observed (Figure 5b). When vesicles, labeled with the gold-capped nanoparticles, were irradiated with CW light at 650 nm (conventional excitation light used for photothermal treatments, using about 0.15 J/cm2), no emitted light could be detected; however, the gold shells strongly absorb and convert light into heat resulting in a drastic alteration of the vesicle structures as documented by the optical image recorded soon after red-irradiation. In particular, DMPC vesicles which are stable in the rigid bilayer-gel form at room temperature, upon irradiation in the SPR band of the gold shell undergo a remarkable structural change. No morphological modifications were observed when the irradiation at 650 nm was carried out on vesicles loaded with the SiO2-AA particles (without the gold shell). This observation was used as control experiment to assign the alteration to plasmonic effects. Furthermore, the heating effects due to the absorption of the gold shells upon irradiation in the red-region was also tested measuring the temperature on Au@ SiO2-AA, aminated SiO2-AA conjugated to gold seeds, and SiO2-AA suspensions when exposed to increasing doses of light in the red-region of the spectrum (650 ( 20 nm). The experiments were carried out in parallel by splitting the light source in three different lines to avoid the effects of the possible source fluctuations. Upon irradiation at 650 nm, the temperature increased in all the samples (Figure 6), but the changes were smaller for the SiO2-AA suspension and for the sample containing the aminated SiO2-AA conjugated to gold seeds. A temperature increase of 10 °C was detected when the sample with a complete gold shell (Au@ SiO2-AA) was irradiated (Figure 6). In agreement with literature data, the larger effect observed for the Au@ SiO2-AA compared to the seed conjugated nanoparticles is likely due to the differences in light absorption efficiencies and not to the yields in heat release.6b,11e It has also to be noted that the nanoshell suspension temperature reached values above the transition temperature of DMPC (28 °C) thus accounting for the structural modification of the vesicles. It is likely that the efficient conversion of 650 nm light into heat by the metal nanostructures leads to a local temperature increase in the system, as recently observed for similar systems;6b when the temperature reaches a value above the transition temperature of the DMPC vesicles, they are in a fluid phase that is characterized by a higher mobility/permeability. In these 21102
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Figure 6. Temperature profile as function of light dose (λirr = 650 nm) of SiO2-AA (cicle), SiO2-AA conjugated to gold seeds (square), and Au@SiO2-AA (star).
conditions the penetration of nanoparticles in the bilayer can be enhanced thus assisting the conformation change of phospholipids leading to the structural change.29 A deeper understanding of the mechanism is currently under investigation. The results indicate that the present engineered nanoparticles have a dual functionality (i.e., fluorescence and photon-to-heat conversion), which can be selectively activated by switching the excitation wavelength. Such multicomponent nanoshells can play an important role in a number of emerging in vivo optical imaging techniques since they allow the integration of diagnostic and therapeutic steps in a single session. Furthermore the optimization of the preparation procedure for multicomponent nanoparticles preserving the fluorescence properties of the dye opens up the application of such nanomaterials in other fields.
4. CONCLUSIONS Hybrid nanoparticles were designed and prepared with the aim to obtain nanomaterials with different independent functions, such as fluorescence and photothermal effect, to be optically activated. In particular, core shell nanostructures based on fluorescent silica nuclei and gold shells were synthesized by a two-step procedure. The fluorescent nuclei were prepared through the sol gel method to achieve silica particles doped with AA molecules. The control of the condensation reaction and the dye loading makes possible to prepare fluorescent silica nuclei with a mean diameter of 31 nm without altering the spectral properties of the dye; a large portion of the fluorescence appeared stabilized upon inclusion in the silica matrix. In the second preparation step, the silica nanoparticles were capped with a thin gold shell. The red shift of the metal plasmon bands was followed to control the shell growth whose morphology and dimension (about 7 nm thick) were analyzed by TEM imaging. The fluorescence of the entrapped dye molecules was not quenched by the metal layer growth, but only a 10 nm blue shift of the spectrum was observed, which was similar to the solvent effects observed in similar systems. Time-resolved fluorescence measurements indicate that the fluorescence of the embedded dye molecules is further stabilized by the metal layer growth, thus supporting the idea that no strong interactions are occurring between the metal and the organic dye. The selective and independent photoexcitation of fluorescence or plasmon absorption has been tested on phospholipidic membranes loaded with the
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prepared hybrid particles. Under visible irradiation fluorescence is activated and used to record fluorescence images of the membranes, without any morphological modifications. Upon irradiation at 650 nm only the gold layers absorb (hence no fluorescence is produced) and efficiently convert light into heat leading to a local heating of the phospholipidic bilayer which resulted in an evident alteration of the membrane architecture. The assignment of this modification to a plasmonic effect has been supported by the lack of observing any changes upon irradiation at wavelengths where gold nanostructures do not absorb (i.e., 400 nm) or upon irradiation in the red region in the absence of gold structures. Furthermore the temperature gradient detected in the Au@ SiO2-AA suspension with increasing the 650 nm light dose confirmed that the alteration is due to photothermal effects. The careful choice of the organic dye, the optimization of the silica nuclei loading and the control of the metal layer growth enable to completely uncouple the fluorescence and the plasmon bands of gold, opening up the possibility to use these engineered nanoparticles for theranostic applications.
’ AUTHOR INFORMATION Corresponding Author
*Phone: ++39-75-5855636. Fax: ++39-75-5855598. E-mail:
[email protected].
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