Contrasting properties of gold nanoparticles for optical coherence tomography: phantom, in

vivo studies and Monte Carlo simulation

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Phys. Med. Biol. 53 (2008) 4995–5009 doi:10.1088/0031-9155/53/18/010

Contrasting properties of gold nanoparticles foroptical coherence tomography: phantom, in vivostudies and Monte Carlo simulation

E V Zagaynova1, M V Shirmanova1,2, M Yu Kirillin3,4, B N Khlebtsov5,A G Orlova1,4, I V Balalaeva2,4, M A Sirotkina1, M L Bugrova1,P D Agrba4 and V A Kamensky4

1 Nizhny Novgorod State Medical Academy, 603005, Minin and Pozharsky square, 10/1,Nizhny Novgorod, Russia2 NI Lobachevsky State University of Nizhny Novgorod, 603950 Gagarin Street, 23,Nizhny Novgorod, Russia3 Optoelectronics and Measurement Techniques Laboratory, University of Oulu, PO Box 4500,90014 University of Oulu, Oulu, Finland4 Institute of Applied Physics of RAS, 603950 Ulyanov Street, 46, Nizhny Novgorod, Russia5 Institute of Physiology and Biochemistry of Plants of RAS, 410049, Entusiastov Avenue, 13,Saratov, Russia

Received 20 March 2008, in final form 17 June 2008Published 18 August 2008Online at stacks.iop.org/PMB/53/4995

AbstractThe possibility of using silica–gold nanoshells with 150 nm silica core size and25 nm thick gold shell as contrasting agents for optical coherence tomography(OCT) is analyzed. Experiments on agar biotissue phantoms showed thatthe penetration of nanoshells into the phantoms increases the intensity ofthe optical coherence tomography (OCT) signal and the brightness of thecorresponding areas of the OCT image. In vivo experiments on rabbitskin demonstrated that the application of nanoshells onto the skin providessignificant contrasting of the borders between the areas containing nanoshellsand those without. This effect of nanoshells on skin in vivo is manifestedby the increase in intensity of the OCT signal in superficial parts of the skin,boundary contrast between superficial and deep dermis and contrast of hairfollicles and glands. The presence of nanoshells in the skin was confirmed byelectron microscopy. Monte Carlo simulations of OCT images confirmed thepossibility of contrasting skin-layer borders and structures by the applicationof gold nanoshells. The Monte Carlo simulations were performed for two skinmodels and exhibit effects of nanoparticles similar to those obtained in theexperimental part of the study, thus proving that the effects originate exactlyfrom the presence of nanoparticles.

(Some figures in this article are in colour only in the electronic version)

0031-9155/08/184995+15$30.00 © 2008 Institute of Physics and Engineering in Medicine Printed in the UK 4995

4996 E V Zagaynova et al

1. Introduction

Optical coherence tomography (OCT) is a noninvasive technique based on low-coherenceinterferometry utilizing backscattered light for imaging biotissues at depths up to 2 mm witha spatial resolution of 5–15 µm at the wavelength from the near-infrared (NIR) range (λ =0.75, . . . ,1.3 µm). In recent years, OCT has proved to be an efficient technique for in vivoimaging of layered tissues: skin and mucous membranes (Fujimoto et al 2000, Feldchtein et al2003, Jackle et al 2000, Zagaynova et al 2002, Dolin et al 2004). Traditionally, osmoticallyactive immersion liquids such as glycerol, propylene glycol, dextranes and concentratedglucose solution are employed for contrasting OCT images (Larina et al 2008, Proskurin andMeglinski 2007, Wang and Elder 2002). The efficiency of using gold nanoparticles as contrastagents for optical imaging of cells and tissue phantoms has been demonstrated (Loo et al2005, Rahman et al 2005), in particular, for nanoshells with core/shell structure. However,available in vivo experimental data are insufficient for developing an appropriate contrastingprocedure.

The advantages of the gold nanoparticles with core/shell structure are low toxicity and anability for maintenance of localized surface plasmon resonances in the NIR region providingenhanced backscattering of laser radiation (Xia and Halas 2005). The nanoshells with silicacore and gold shell seem to be the most perspective for contrasting OCT images of biotissuesbecause the wavelengths of plasmon resonances they maintain are in the so-called transparencywindow (λ= 0.6, . . . , 1.3 µm), within which the OCT operating wavelength is usually situated.By varying the geometrical parameters of such nanoparticles one can adjust the backscatteringmaximum for a given wavelength and obtain particles with preset optical properties (Loo et al2004).

Being effective for backscattering imaging, nanoparticles are potential effectivecontrasting agents for OCT imaging of skin. The skin is known to have a complex-layeredstructure with different optical properties of the layers. Multiple light scattering in skinoriginating from its optical non-uniformity significantly limits the contrasting of layers, theseparation of structures within skin, as well as the imaging depth of OCT (Gladkova et al 2000).Unfortunately, the contribution of multiple scattering bringing distortions and blurring cannotbe separated from the experimental OCT image. In order to analyze the role of multiplescattering and give an interpretation to the obtained OCT images theoretical or numericalsimulations are required. The numerical Monte Carlo method has been shown to be an effectivetool for the simulation of OCT signals from multilayer media in our earlier papers (Kirillinet al 2005, 2006) and was later used for the simulation of OCT images of multiplayer skinphantoms (Kirillin et al 2007).

The goal of the present study is to evaluate the efficiency of silica–gold nanoshells ascontrasting agents in OCT imaging of phantoms and tissues and to interpret the obtainedexperimental images by comparison with the results of Monte Carlo simulations.

2. Materials and methods

2.1. OCT system

The OCT system developed at the Institute of Applied Physics of the Russian Academy ofSciences equipped with a flexible probe described by Gelikonov et al (2003) was used in thepresent study. The system has the following characteristics: the outer diameter of the probe is2.7 mm, the operating wavelength is 900 nm, the probing radiation power is 2 mW, the spatialresolution in air is about 15 µm and the average time required for obtaining a 2D image of

Contrasting properties of gold nanoparticles for optical coherence tomography 4997

Figure 1. Schematic diagram of the OCT probe position: 1—output lens, 2—output glass window(its right boundary is the OCT probe surface), 3—sample.

200 × 200 pixels is 1.5 s. Special software provided with the system creates images in thecolor pattern where brighter areas correspond to the higher intensities of the backscatteredlight, while the darker ones to the lower intensities. The OCT probe was positioned on theskin surface perpendicularly, with uniform pressure distribution over the area under study.The schematic of the probe position relative to the sample under study is presented in figure 1.

2.2. Silica–gold nanoshells

We studied the influence of silica–gold nanoshells with 150 nm diameter silica core and25 nm thick gold shell on the optical properties of biotissues. Silica–gold nanoshells werefabricated as described by Oldenburg et al (1998) with minor modifications in the reagentconcentrations. The following reagents were used in the synthetic procedures: tetraethylorthosilicate (TEOS; Aldrich), 3-aminopropyltrimethoxysilane (APTMS; Aldrich), 3-mercaptopropyltrimethoxysilane (MPTES; Aldrich), tetrakis (hydroxymethyl) phosphoniumchloride (THPC; Fluka), Tetrachloroauric acid (TCAA; Aldrich), potash (Reachim Co.,Russia) and formaldehyde (Serva) were of research grade; 25% aqua ammonia was ofanalytical grade. Ethanol was purified by additional distillation.

First, silica nanoparticles were grown by reducing TEOS with NH4OH in absolute ethanol.For the preparation of gold seeds, 220 µl of 1 M aqueous NaOH and 6 µl of 80% THPC wereadded to 20 ml of triply distilled water. The solution was vigorously agitated on a magneticstirrer at 1000 rpm, and 880 µl of a 1% TCAA solution were added.

Next, aminated silica particles were added to the gold-seed suspension. Gold particlesadsorb to the amine groups on the silica surface, resulting in a silica nanoparticle coveredwith the gold colloid. Silica–gold nanoshells were then grown by reacting HAuCl4 with thesilica–colloid particles in the presence of formaldehyde at room temperature. This processreduces additional gold on the adsorbed colloid, which act as nucleation sites. The nanoshellswere centrifuged and sonicated in HPLC-grade water until use. The concentration of thenanoshells was determined to be 5 × 1010 particles ml−1.

The gold nanoshells with the above sizes have backscattering maximum in the NIR region(850–950 nm). It is well known that gold nanoshells with shell thickness more than 20 nmare more scattering than absorbing (Xia and Halas 2005), i.e. their optical characteristicsare optimal for OCT imaging (Gobin et al 2007). The discussion about how the optimalnanoparticles size depend on the wavelength is presented in Lin et al (2005).

An electron micrograph and an extinction spectrum of the silica–gold nanoshells areshown in figure 2.

2.3. Biotissue phantom

One- and two-layer biotissue phantoms created from agar gel of different concentrations (from0.2% to 1%) were used in the preliminary study. The gels were manufactured according to the

4998 E V Zagaynova et al

(a) (b)

Figure 2. The electron micrograph (a) and the extinction spectrum (b) of 150/25 gold nanoshells.

technique described by Gaal et al (1980). As was shown there, the size of the pores obtainedinversely depends on the agar concentration. For the concentrations varying from 0.2% to 1%the average size of the pores varies from 530 nm to 85 nm (0.2%: 530 nm, 0.3%: 343 nm,0.5%: 200 nm, 1%: 85 nm). A colloidal solution of gold nanoshells was applied once on thephantom surface. An OCT probe was fixed 0.2–0.3 mm over the gel surface. OCT imageswere obtained every 30 min during 5 h after application, then once a day. The maximumobservation time was 48 h. The OCT images of the biotissue phantom before the applicationof gold nanoshells were taken for reference.

2.4. Animals tests in vivo

Further we analyzed a possibility of using nanoparticles as contrast agents for OCT in vivo.The solutions were applied on healthy skin areas on the thighs of 3–3.5 kg rabbits afterdepilation.

As the first step in our experiments, the effects of silica–gold nanoshells after oneapplication on the skin surface were studied using OCT. The OCT probe was placed directlyonto the surface of the animal skin with equal pressure at each point. The first OCT imagewas acquired from a native skin site as a reference before the application of nanoparticles, thesecond OCT image was obtained immediately upon the application of the chemical agent andsubsequent images were taken every 30 min during 4.5 h and then after 24 h. Further, westudied the effects of multiple applications of nanoshells on skin with 30 min intervals. Skinlayers in the OCT images were identified by comparison with histology data. The presence ofgold nanoshells in the skin structure was confirmed by electron microscopy.

2.5. Monte Carlo simulation of OCT images

Monte Carlo (MC) simulation of light propagation is based on the calculation of a large numberof trajectories of photons randomly propagating in a scattering medium (Wang et al 1995)and further statistical analysis of the obtained data. The optical properties of the medium(scattering and absorption coefficients, phase function or anisotropy factor, refractive index)determine the lengths and shapes of individual photon trajectories. In the current work, weused the program code of the MC algorithm developed earlier by Kirillin et al (2005, 2006 and2007) for the simulation of OCT signals and images. We employed the Henyey–Greenstein

Contrasting properties of gold nanoparticles for optical coherence tomography 4999

Table 1. Optical properties of thick skin layers (λ = 900 nm).

Skin layer Thickness (mm) µs (mm−1) µa (mm−1) g n

Upper stratum corneum 0.02 35 0.02 0.9 1.54Lower stratum corneum 0.08 5 0.015 0.95 1.34Epidermis 0.2 12 0.02 0.85 1.4Dermis with upper plexus 0.2 12 0.1 0.9 1.39Dermis 0.8 7 0.7 0.87 1.4Dermis with lower plexus 0.6 12 0.2 0.95 1.39

Table 2. Optical properties of thin skin layers (λ = 900 nm).

Skin layer Thickness (mm) µs (mm−1) µa (mm−1) g n

Stratum corneum + epidermis 0.015 35 0.02 0.9 1.45Dermis with upper plexus 0.15/0.25 12 0.1 0.9 1.39Dermis 0.5 7 0.7 0.87 1.4

phase function that is widely used for MC simulations of light propagation in biotissuesincluding skin (Tuchin 2000),

fHG(θ) = 1

4π

(1 − g2)

[1 + g2 − 2g cos(θ)]3/2,

where θ is the scattering angle and g is the anisotropy factor.For the simulation of the 2D OCT images the consequent OCT A-scans are simulated with

a definite step in the probing position. The total number of A-scans and the step between themare preset. The step is usually chosen as a width (FWHM) of the probing beam diameter. Thecoherence length in the simulations was taken to be lcoh = 15 µm. For calculating a 2D OCTimage, 50 A-scans with a transversal step of 20 µm were calculated. A typical calculationof an OCT image of human skin takes about 10 h on a PC with an AMD AthlonTM 3000processor.

2.6. Multilayer model of skin

Two skin models were utilized in the simulations. It is well known that human skin issubdivided into two types based on the thickness of the stratum corneum and the anatomicalplace. The first one is thick skin that has a very good representative stratum corneum and issituated on palms and plants. The second one is thin skin that has a small stratum corneum andcovers the remaining body surface (Damjanov 1996). Due to the different optical propertiesof different types of skin we applied different schemes for each. One of them, a six-layer skinmodel corresponding to thick human skin was utilized by Kirillin et al (2007) for a numericalanalysis of OCT signal formation, another one, a three-layer model corresponding to thin skinis based on the structure of the rabbit skin samples used in the experimental part of the presentstudy. The schematics of the models for thick and thin skin are presented in figures 3(a) and(b), respectively.

The optical properties of thick and thin skin layers used in the simulations are listed intables 1 and 2, respectively. These values are chosen based on the data obtained by Tuchinet al (2000) and Knuttel and Boehlau-Godau (2000), so that the obtained simulated OCTimages qualitatively agree with the experimental ones.

The scattering by skin forming elements is supposed to be characterized by theHenyey–Greenstein phase function, while the scattering on nanoparticles embedded into

5000 E V Zagaynova et al

0.0 0.2 0.4 0.6 0.8 1.0

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

Z, m

m

X, mm

Upper stratum corneumLower stratum corneumEpidermisDermis with upper plexus

Dermis

Dermis with lower plexus

0.0 0.2 0.4 0.6 0.8 1.0

0.80

0.75

0.70

0.65

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

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0.15

0.10

0.05

0.00

Stratum corneum + epidermis

Dermis with upper plexus

DermisZ, m

m

X, mm

(a)

(b)

Figure 3. Six-layer thick skin model (a) and three-layer thin skin model (b) used in the MonteCarlo simulation.

skin is supposed to be isotropic. The layers are supposed to contain uniformly distributednanoparticles or not to contain them at all. The effect of nanoparticles was simulated byintroducing a certain probability for a photon to experience scattering on a nanoparticle,when it propagates in the layer containing nanoparticles. This probability depends ona predefined nanoparticles concentration which was chosen so that it provides the partialscattering coefficient µ

ps equal to 1 mm−1.

3. Results and discussion

3.1. Biotissue phantom results

In experiments with one-layer agar gels, OCT showed the dynamics of penetration of goldnanoshells into a porous medium. We detected this penetration in the phantom as an increase

Contrasting properties of gold nanoparticles for optical coherence tomography 5001

(a) (b) (c) (d)

Figure 4. OCT images of the one-layer biotissue phantom at low agar concentration (0.2%): (a)before application, (b) 60 min, (c) 150 min and (d) 17 h after the application of gold nanoshells.1—OCT probe surface, 2—area between the OCT probe and the phantom surfaces, 3—biotissuephantom. The white bar corresponds to 1 mm.

(a) (b) (c) (d)

Figure 5. OCT images of the one-layer biotissue phantom with an agar concentration of 0.5%:(a) before application, (b) 60 min, (c) 17 h and (d) 45 h after the application of gold nanoshells.

of the intensity of the OCT signal from the upper part of the sample resulting in the appearanceof a bright layer in the OCT images. The thickness of this bright layer increased proportionallyto the penetration speed of the gold nanoshells. In the case of low-concentration agar gels,i.e., gels with large pores, the diffusion rate of the gold nanoshells was high. The diffusionof gold nanoshells into the low-concentrated one-layer agar gel is shown in figure 4. One cansee that during 3 h the nanoshells penetrate at depths over 1 mm.

The diffusion rate was lower in gels with higher agar concentrations. For instance, for anagar concentration of 0.5%, the nanoshells penetrated to the depth of 1 mm in 45 h (figure 5).

In the case of highest agar concentration (1%) and very small pores, they stay on thephantom surface and practically do not penetrate inside the medium.

Two-layer biotissue phantoms allowed us to study the process of boundary contrasting.In the case of low-concentration agar in the upper layer and high concentration in the deeperphantom layer, after the application of gold nanoshells one can see a pronounced contrast ofboundary due to the presence of the nanoshells in the upper layer and their absence in thedeeper one (figure 6).

Thus, we have shown that the presence of gold nanoshells in certain tissue phantom areasincreases the level of the OCT signal from these areas and, consequently, the brightness ofthe corresponding areas in the OCT images. In a two-layer phantom with different diffusionproperties, gold nanoshells provide pronounced boundary contrasting.

3.2. Animals test results

At the next stage of the study, we acquired in vivo OCT images of healthy skin regions inrabbits before and after the application of a colloidal solution of gold nanoshells. In OCT

5002 E V Zagaynova et al

(a) (b)

Figure 6. OCT images of the two-layer biotissue phantom with an agar concentration of 0.3% inthe upper layer and 0.5% in the lower layer: (a) before application, (b) in 44 h of gold nanoshells.1—OCT probe surface, 2—upper phantom layer, 3—lower phantom layer.

(b)

(a)

(c)

100 µm

Figure 7. Histology samples of rabbit thigh skin (a): 1—epidermis, 2—superficial part of thedermis with hair bulbs, 3—deep part of the dermis. OCT images of the rabbit skin beforeapplication (b) and 3 h (c) after the application of silica–gold nanoshells. The arrow indicates theborder between the superficial part and the deep part of the dermis. The area of hair follicle isindicated by the dashed line.

images of skin without nanoshells it was difficult, as a rule, to separate tissue layers andstructures. In this case, depth localization of layers and structures can be determined only bycomparing the data of OCT images with the corresponding histology samples (figures 7(a)and (b)).

It was found that a single application of a colloidal solution of silica–gold nanoshellson skin causes a number of changes in the OCT images (figure 7(c)). The application ofgold nanoshells leads to an increase in the intensity of the OCT signal in the areas containingnanoparticles causing an increase in brightness of the superficial part of the dermis andcontrast between the superficial and deep parts of the dermis in 30 min after the application.Typically, in 3 h the changes in OCT images become more pronounced as the brightness of

Contrasting properties of gold nanoparticles for optical coherence tomography 5003

Figure 8. Electron micrograph of rabbit skin 3 h after the application of gold nanoshells.Nanoparticles at the border with the epithelial cell nucleus are shown by the arrows. Magnification×11000.

Figure 9. Averaged A-scans of the skin. Solid line—A-scan after the application of goldnanoshells; dashed line—control without gold nanoshells. The arrow indicates the region ofthe OCT signal drop at the border between the superficial and deep parts of the dermis.

the superficial part of the dermis and the contrast between the superficial and deep parts ofthe dermis continue to increase. In addition, the border between the superficial and deep partsof the dermis becomes more distinct, continuous and well discernible, allowing to accuratelydifferentiate these layers. Besides, the application of silica–gold nanoshells causes contrastingof hair follicles and glands in the OCT images. The contrasting effects of the gold nanoshellslasted up to 24 h of observation.

The penetration of nanoparticles into the upper part of the skin (epidermis, upper partof dermis) was confirmed by electron microscopy. In skin samples taken 3 h after colloidalsolution application, nanoshells were found in large amounts in the epidermis and dermis,inside cells and intercellular substance. No nanoshells were found in hair bulbs. An electronmicrograph of the skin with gold nanoshells is given in figure 8.

Analysis of separate A-scans confirmed the observed contrasting effects of gold nanoshells(figure 9). An averaged A-scan of the skin after the application of gold nanoshells shows a

5004 E V Zagaynova et al

Figure 10. Effects of multiple applications of gold nanoshells. OCT image of rabbit skin after thefourth application. Arrows indicate areas of gold nanoshells accumulation on the skin surface.

higher intensity of the OCT signal in the upper skin layer in comparison with the referenceimage without nanoshells. A drop in the intensity of the OCT signal at a depth of about400 µm provides pronounced contrast between superficial and deep dermis on the OCTimage.

In order to evaluate the optimal gold nanoshells concentration, a solution of goldnanoshells was applied repeatedly on the surface of rabbit skin in vivo. A drop of thegold nanoshell solution was applied five times in a row every 30 min. OCT images weretaken before and immediately after each application. The OCT image of skin after multiplenanoparticle application is shown in figure 10. It was found that the multiple applicationsdo not lead to any additional increase in the contrast of the OCT images. After the secondand third applications a decrease in the imaging depth was noted, while the higher intensityof the OCT signal in the superficial layer of skin was preserved. As a result of furtherapplications, the level of the useful signal became lower and dark vertical strips appearedin the OCT images. We suppose that multiple applications resulted in the accumulation ofgold nanoshells in irregularities on the skin surface, thus screening the OCT signal fromdeeper tissue layers and resulting in bright areas in the superficial layers and dark strips belowthem.

3.3. Monte Carlo simulation results

Monte Carlo simulations of the OCT images of skin before and after the application ofnanoparticles were performed. The aim of this study was to prove that the contrastingeffect observed in the experiment is due to the presence of nanoparticles rather than to otherchanges in skin properties caused by their application. Towards this end, only the influenceof nanoparticles was taken into account in the simulations.

At the first stage, simulations were made for the six-layer model of thick skin. Thenanoparticles were added to the skin model layer by layer starting from the superficial stratumcorneum. The results of these simulations are presented in figure 11.

The results of the simulations show that the application of nanoparticles provides thecontrasting of the bottom boundary of the deepest layer containing nanoparticles. This effectis similar to that obtained in the experiment where the boundary of the superficial layer ofdermis was contrasted.

Another goal of the modeling was to simulate the effect of the dark stripes observed inthe OCT images when the areas of higher concentration of nanoparticles are present in thesuperficial layers of skin (figure 10). To achieve this goal, four areas where the concentrationof nanoparticles was three times higher compared to the other areas were allocated in the

Contrasting properties of gold nanoparticles for optical coherence tomography 5005

(a) (b)

(c) (d)

Figure 11. Simulated OCT images of thick skin without nanoparticles (a), with nanoparticles inthe upper two layers (b), in the upper three layers (c) and in all six layers (d). The partial scatteringcoefficient of particles µ

ps = 1 mm−1. In the vertical scale and the horizontal scale the optical and

physical distances, respectively, are shown in micrometer.

model skin. These areas were supposed to be present in the stratum corneum and to be thickerthan the stratum corneum at the other points.

The results of the simulations are presented in figure 12. From this figure one can see thatthe simulation results exhibit an effect similar to that in the experiment; however, the contrastof the obtained stripes is lower that in the experiments. This fact allows us to concludethat, although the origin of this effect is confirmed qualitatively, the scattering and absorptionproperties of these areas in the experiment are higher than those in simulation.

Another set of simulations was performed using the three-layer model of skin assumingthat the gold nanoparticles penetrate only into two upper layers of the considered skin model—epidermis (including stratum corneum) and dermis with upper plexus. The border betweendermis with upper plexus and dermis was supposed to have a wavy shape and its contrastingwas chosen as a criterion for the effectiveness of the nanoparticles application.

5006 E V Zagaynova et al

Figure 12. Simulated OCT images of skin with nanoparticles in the stratum corneum with fourareas of high concentration of nanoparticles. In the vertical scale and the horizontal scale theoptical and physical distances, respectively, are shown in micrometer.

(a) (b)

Figure 13. Simulated OCT images of skin before the application of gold nanoparticles (a) andin the presence of nanoparticles in two upper layers (b). The concentration of nanoparticles insupposed to provide the partial scattering coefficient µ

ps = 1 mm−1. The thickness of dermis with

upper plexus is 0.15 mm. In the vertical scale and the horizontal scale the optical and physicaldistances, respectively, are shown in micrometer.

The results of the simulations show that in the absence of nanoparticles in the modelskin sample, the border between two dermis layers is not contrasted (figure 13(a)), whilein the presence of nanoparticles in the two upper layers, the border becomes contrasted(figure 13(b)).

The intensity of the OCT signal from the upper layers increases due to the increase inbackscattering from these areas caused by the presence of nanoparticles, while the intensityof the signal from the deeper layer remains the same. This fact causes a significant increasein contrast of the boundary imaging.

OCT images from the skin sample containing a hair bulb were simulated assuming thatthe hair bulb is an area within dermis with the upper plexus layer whose optical propertiescoincide but nanoparticles do not penetrate into this area. The shape of the hair bulb wasconsidered to be spherical. The simulations were performed for two values of the hair bulb

Contrasting properties of gold nanoparticles for optical coherence tomography 5007

(a) (b)

Figure 14. Simulated OCT images of skin containing a hair bulb with the radius of 0.07 mm(a) and 0.12 mm (b) in the presence of nanoparticles in two upper layers. The concentration ofnanoparticles is supposed to provide the partial scattering coefficient µ

ps = 1 mm−1. The thickness

of the dermis with upper plexus is 0.25 mm. In the vertical scale and the horizontal scale theoptical and physical distances, respectively, are shown in micrometer.

radius, 0.07 mm and 0.12 mm, while the thickness of dermis with upper plexus layer wastaken to be 0.25 mm. The results of the simulations are presented in figure 14.

From this figure one can see that the hair bulb containing no gold nanoparticles is acontrasting structure against the background of the dermis layer with upper plexus containingnanoparticles.

Thus, Monte Carlo simulation confirmed that nanoparticles introduce additional opticaleffects to the OCT images. An essential effect is an increase of the level of the signalfrom the areas containing nanoparticles, providing contrast with layers and structures withoutnanoparticles.

4. Conclusion

In the present work, we have demonstrated the efficiency of the silica–gold nanoshells ascontrasting agents for OCT imaging of agar biotissue phantoms and rabbit skin in vivo.

Experiments on agar biotissue phantoms showed that when silica–gold nanoshellspenetrate into the phantoms, and increase the intensity of the OCT signal from the areasthey present within. In the case of a two-layer phantom, when the diffusive properties of thelayers differ, in a certain moment of time the nanoshells concentrate on the boundary betweenthe layers and provide strong contrasting of this boundary.

In vivo experiments on rabbit skin demonstrated that the application of silica–goldnanoshells on a skin sample provides significant contrasting of the boundaries between theareas containing nanoshells and those without. The effect of gold nanoshells on skin in vivo ismanifested by the increase in the intensity of the OCT signal in superficial layers of the skin(epidermis, superficial dermis), boundary contrast between superficial and deep dermis andcontrast of hair follicles and glands. The presence of nanoshells in the skin layers and theirabsence in hair follicles and glands was confirmed by electron microscopy.

Monte Carlo simulations of OCT images confirmed the possibility of contrasting skin-layer boundaries and constituent structures by the application of gold nanoshells, thus provingthat the effects originate exactly from the presence of nanoparticles. The Monte Carlosimulations were performed for two skin models and exhibit the following contrasting effects

5008 E V Zagaynova et al

of nanoparticles: contrasting of the boundaries between skin layers, contrasting of a hairfollicle and the effect of dark stripes originated from the high concentration of nanoshells onthe skin surface.

Acknowledgments

This work was supported in part by the Science and Innovations Federal Russian Agency(projects no 02.522.11.2004, 02.512.12.2244). Mikhail Kirillin would like to acknowledgeGETA Graduate School and Tauno Tonning Foundation (Finland) for financial support. Theauthors are grateful to L B Snopova (Nizhny Novgorod State Medical Academy) for helpin performing the microscopical analysis procedure. Also the authors thank the Institute ofBiochemistry and Physiology of Plants for providing silica–gold nanoshells.

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