nanoscalebiomoleculardetectionlimitforgoldnanoparticles ...it oﬀers real-time in situ analysis of...

Hindawi Publishing CorporationAdvances in Optical TechnologiesVolume 2012, Article ID 278194, 8 pagesdoi:10.1155/2012/278194

Research Article

Nanoscale Biomolecular Detection Limit for Gold NanoparticlesBased on Near-Infrared Response

Mario D’Acunto,1, 2 Davide Moroni,2 and Ovidio Salvetti2

1 Istituto di Struttura della Materia, Consiglio Nazionale delle Ricerche, Via Fosso del Cavaliere 100, 00133 Roma, Italy2 Istituto di Scienze e Tecnologia dell’Informazione, Consiglio Nazionale delle Ricerche, Via Moruzzi 1, 56124 Pisa, Italy

Correspondence should be addressed to Mario D’Acunto, [email protected]

Received 27 July 2012; Accepted 31 October 2012

Academic Editor: Carlo Corsi

Copyright © 2012 Mario D’Acunto et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Gold nanoparticles have been widely used during the past few years in various technical and biomedical applications. In particular,the resonance optical properties of nanometer-sized particles have been employed to design biochips and biosensors used asanalytical tools. The optical properties of nonfunctionalized gold nanoparticles and core-gold nanoshells play a crucial role forthe design of biosensors where gold surface is used as a sensing component. Gold nanoparticles exhibit excellent optical tunabilityat visible and near-infrared frequencies leading to sharp peaks in their spectral extinction. In this paper, we study how the opticalproperties of gold nanoparticles and core-gold nanoshells are changed as a function of different sizes, shapes, composition, andbiomolecular coating with characteristic shifts towards the near-infrared region. We show that the optical tenability can be carefullytailored for particle sizes falling in the range 100–150 nm. The results should improve the design of sensors working at the detectionlimit.

1. Introduction

The development of biosensors devices requires sophisti-cated approaches to detect and to identify the analytes[1–3]. Because the sensor signal-to-noise ratio increaseswith decreasing size for many devices, many researchersare expending considerable effort for miniaturizing sensingdevices down to nanoscale size [3–7]. In the past decadeseveral authors have analyzed the effect of flow, size, andadsorption isotherms on biomolecular adsorption and ina limited way the effects at nanometer length scales. Therole of nanoscale size can be shown examining a simplesensor geometry (e.g., hemisphere) that absorbs the analyte[6, 8, 9]. The maximum number of molecules, N(t), that canaccumulate on a sensor due to irreversible adsorption may bedetermined by

N(t) =∫ t

0F(τ)dτ =

∫ t

0

∫Af dσdτ, (1)

where F is the total flux (molecule s−1), f is the flux at thesensors (molecules s−1m−2), σ is the unit area, A is the sensor

area, and t is time. Time dependence in (1) involves a crucialquestion for clinical applications; that is, if any analytemolecule that contacts the sensor surface can be detected,what is the minimum detectable concentration for a givenaccumulation time? Sheehan and Whitman [6] showed thatDNA microarrays with a 200 μm diameter hemisphere candetect ∼1 fM in ∼1 min, which is actually very close to theactual DNA detection limit, ∼20 fM. Analogously, from (1)is possible to deduce the accumulation events

N(t) = 4DNAc0at, (2)

where NA is Avogadro’s number, c0 is the initial solutionconcentration, a is the radius of the sensing surface, and D isa diffusion constant. It is remarkable that (2) is linear in bothradius and time. This implies that when reducing the size ofthe absorbing surface, the time for a critical accumulation ofevents increases consequently. The advent of nanotubes andnanowires as biosensors introduces new geometries into thesensing activity with some changes to accumulation events asin (2).

2 Advances in Optical Technologies

The use of nanotechnologies for diagnostic applicationsmeets the rigorous demands of clinical standards sensitivitywith cost effectiveness. Today, main nanodiagnostic toolsinclude quantum dots (QDs), gold nanoparticles, and can-tilevers [10]. The effectiveness of nanoparticles as biomedicalimaging contrast and therapeutic agents depends on theiroptical properties. Biosensing applications based on surfaceplasmon resonance shifts need strong resonance in thewavelength sensitivity range of the instrument as well asnarrow optical resonance line widths. For actual in vivoimaging and therapeutic applications, the optical resonanceof the nanoparticles is strongly desired to be in the near-infrared (NIR) region of the biological water window, wherethe tissue transmissivity is the highest [11–15].

The optical properties of gold nanoparticles in thevisible and near-infrared (vis-NIR) domains are governedby the collective response of conduction electrons. Theseform an electron gas that moves away from its equilibriumposition when perturbed by an external light field, thuscreating induced surface polarization changes that act as arestoring force on the electron gas. This results in a collectiveoscillatory motion of the electrons similar to the vibrationsof a plasma and characterized by a dominant resonance bandlying in the vis-NIR for gold and called plasmon excitations.Thus the surface of gold nanoparticles can be used as asensing element because when biomolecules attach to suchsurface the binding event can be revealed by optical changes.In addition, it is possible to calculate the maximum numberof possible binding events and the consequent opticalresponse when the shape and size of the gold particles areknown.

In this paper, we will calculate and simulate the opticaltunability of gold nanoparticles and core-gold nanoshells(optical changes with nanoparticles sizes) using the classicalMie theory and discrete dipole approximation (DDA). Wewill show that when biomolecules bind to gold surfaceoptical shifts toward the near-infrared region are produced,and wavelengths changes can be accurately quantified. Theaccurate knowledge of the optical tunability could allow forimproving sensing performance, sensitivity, and figure ofmerit of biosensors operating at detection limit in short timeas requested in clinical applications.

2. Nanoscale-Sized Systems for Detection Limit

The increased demand for sensitivity requires that a diag-nostically significant interaction occurs between analytemolecules and signal-generating particles, thus enablingdetection of a single analyte molecule. Nanotechnology hasenabled one-to-one interaction between analytes and signal-generating particles such as QDs and gold nanoparticles.Here, we review briefly the most important nanoscale toolsfor detection limit, such as QDs, cantilevers, and goldnanoparticles [10, 16].

QDs are semiconductor nanocrystals, characterized bystrong light absorbance, that can be used as fluorescent labelsfor biomolecules. A typical QD has a diameter of 2–10 nm

and is usually composed of a core consisting of a semicon-ductor material enclosed in a shell of another semiconductormaterial with a larger spectral bandgap. When a QD absorbsa photon with energy higher than the bandgap energy of thecomposing semiconductor, an exciton (electron-hole pair) iscreated. As a result, a broadband absorption spectrum occursbecause of the increased probability of absorption at shorterwavelengths. The recomposition of the exciton, generallycharacterized by a long lifetime >10 ns, to a lower energystate leads to emission of a photon with a narrow symmetricband [17–19]. QDs are reported to emit with lifetimes of 5–40 ns, whereas conventional dyes emit in a time less than 2 ns.This characteristic produces a strong and stable fluorescencesignal. In addition, due to quantum confinement effect adirect relationship exists between the QD size and the valuesof the quantized energy levels. Methods for tracking anddetecting QDs are numerous and include fluorometry andseveral types of microscopy, such as fluorescence, confocal,total internal reflection, wide-field epifluorescence, near-field optical microscopy, and multiphoton microscopy. Thechoice of detection techniques depends on the emission ofwavelengths; for example, QDs composed by CdSe/NzS emitin the 530–630 nm range and InP and InAs QDs emit in theNIR range, while PbS emits in the 850–950 nm range andPbSe QD in the mid-infrared range.

Another nanoscale detection technique is based oncantilevers [20–22]. Cantilevers are small beams similar tothose used in atomic force microscopy and they operatedetection by use of nanomechanical deflections. An instruc-tive example of operating cantilevers is the detection ofDNA hybridization. The cantilever surface holds a particularDNA sequence capable of binding to a specific target. Whenthe hybridization occurs with the cantilever single-strandedDNA, mechanical stress produced by the rebinding processunderlying the hybridization deflects the cantilever, with ameasurable deflection by the optical lever method propor-tional to the amount of DNA hybridized. This techniquecan be used as a microarray, allowing for multiple analyses.However, this method currently needs further developmentto solve the problem of nonspecific binding [23].

Gold nanoparticles and gold nanoshells provide greatsensitivity for the detection of DNA antibodies and proteins[24–28]. The instrumental platform for the detection ofbiomolecules including gold particles is based on surfaceplasmon resonance (SPR). SPR is an optical techniquethat measures the refractive index of very thin layers ofmaterial adsorbed on a metal. It offers real-time in situanalysis of dynamic surface events and is capable of definingrates of adsorption and desorption for surface interactions.Plasmon-plasmon resonance, resulting from the interactionof locally adjacent gold nanoparticle labels that have boundto a target, produces changes in optical properties that canbe used for detection. It is known that the characteristicred color of gold colloid changes to a bluish-purple coloron colloid aggregation because of this effect. Analogously,the reduction of gold particle sizes involves shifts toward theNIR region [29]. Raman spectroscopy is a favored detectionmethod using silver in the visualization process. In thiscase, gold nanoparticles can be coated with silver shells;

Advances in Optical Technologies 3

silver-coated gold particles less than 100 nm in size havestrong light-scattering properties and can easily be detectedby optical microscopies operating at the detection limitfor oligonucleotides down to ∼10 fM, that is, nearly 50-fold lower than conventional fluorophore-based methods.Gold nanoshells could allow direct, rapid, and economi-cally feasible analysis of whole blood samples. Generally,a nanoshell consists of concentric spherical nanoparticleswith a dielectric core, meanly consisting of gold sulfideor silica, surrounded by a thin gold shell. Variations inthe relative thickness of the core and outer shell allow theoptical resonance of gold to go into the midinfrared region.Other relative surface properties can be used to focus theabsorption wavelength range into the near-infrared, justabove the absorption of hemoglobin and below that of water[30]. This characteristic aids in avoiding interference fromhemoglobin giving the possibility to do a direct analysis ofwhole blood. An additional advantage of gold nanoshells isthe excellent biocompatibility.

The optical tunability of gold nanoparticles and core-gold nanoshells resides in plasmon resonance properties.It is well known that the plasmon resonance of metalnanoparticles is strongly sensitive to the nanoparticles’ sizeand shape and the dielectric properties of the surroundingenvironments. We will simulate the optical tunability ofgold nanoparticles and gold nanoshells (optical changeswith nanoparticles’ sizes and surface biomolecular coating)using the classical Mie theory and DDA-modified approach.Finally, we will obtain the wavelength shift as a function ofdifferent size dimension, environments, and biomolecularcoating layer of the gold nanoparticles. The accurate knowl-edge of the optical tunability and wavelength shifts shouldallow for development of biosensors operating at detectionlimit in short time as requested in clinical applications.

The design of biosensors working at the detection limitmust attempt to compare sensing performance, sensitivity,and figure of merit (FOM) [31]. The sensitivity S is definedas the ratio of the resonant wavelength shift ∂λres to thevariation of the surrounding refractive index ∂ns, while theFOM is defined as the ratio of the refractive index sensitivityto the resonance width Δλ:

S = ∂λres

∂ns, FOM = S

Δλ. (3)

Resolution is typically defined as the minimum detectionlimit. In addition, sensitivity and thence resolution can bemodified by size and geometry of the gold nanoparticlesand nanostructure environment. In general, the adsorbedmass on the sensor surface can be approximated by the DeFreijter’s formula, which is based on the refractive indexchange [32]

γ = dΔn

∂n/∂c, (4)

where d is the dimension of the adsorbed species(biomolecule) falling in the range of nanometers, Δn isthe refractive index difference between the medium andthe species, and ∂n/∂c is the correspondent biomolecular

refractive increment. Analogously, the spectral response ofthe refractometric nanoplasmonic sensors can be describedby

Δλ = m(neff − nmedium), (5)

where m is the refractive index sensitivity, expressed inplasmon resonance peak shift per refractive index unit (RIU),and neff is the effective refractive index of the adsorbatelayer. In the simplest approximation, the surface plasmonresonance-induced evanescent field decays exponentiallyfrom the surface as E(z) = exp(−z/ld) with a decay lengthld. The effective refractive index of the adsorbate layer isdescribed as

neff = 2ld

∫ d

0n(z)E2(z)dz. (6)

Using (5)-(6) into (4) we obtain a relationship of the surfaceplasmon resonance peak shift with the surface coverage of theadsorbate:

γ(t) = dΔλ(t)m(1− exp(−2d/ld)

)∂n/∂c

. (7)

In the next section, we will provide the accurate calculationof wavelength shifts due to increased size of gold nanopar-ticles and coating of gold surface in core-gold nanoshells.Equation (7) establishes that the accurate knowledge of suchshifts gives the possibility to design gold nanoparticles basedsensors working at the biomolecular detection limit.

3. Optical-Infrared Response of GoldNanoparticles and Core-Gold Nanoshells:Calculation and Simulation Methods

The complexity of the electromagnetic field in the presenceof arbitrarily shaped nanoparticles is such that Maxwell’sequations must be solved using numerical methods [33].The far-field performance of a nanoparticle is summarizedin the wavelength-dependent absorption and scattering crosssections, and as a consequence, the optical properties of goldnanospheres and silica-gold nanoshells will be quantified interms of their calculated absorption and scattering efficiency.The light intensity transmitted through a dilute dispersion I(assuming no more than one photon-particle collision perphoton) is I = I0 exp[−(σabs + σsc)NL], where I0 is theincident light intensity, N is the number of particles perunit volume, L is the path length inside the dispersion, andσabs and σsc are the wavelength-dependent absorption andscattering cross sections, respectively. The optical modelingof nanoparticles thus relies on the solution of Maxwell’sequations for each specific geometry and set of illuminationconditions, assuming a local dielectric function, ε(ω), anddescription of the materials involved. When dipolar contri-bution in plasmon excitation is prominent, then the far-fieldcross sections can be obtained from the polarizability α usingthe following expressions [34]:

σabs + σsc = 2πλ√εm

Im{α}, σsc = 8π3

3λ4|α|2, (8)


where εm is the environmental permittivity of the mediumoutside the particle and λ is the light wavelength.

To simulate the optical properties of gold nanoparticles,many methods can be found. Mie theory is one of suchmethods [35]. Mie scattering theory begins with Maxwell’sequations and the necessary boundary conditions, whichare then transformed into spherical polar coordinates witha solution of the wave vector equation emerging. Thecoefficients for scattering arise and the theory is extended tothe far-field solution based on a plane constructed from theincident and scattered waves. At this point, the Stokes param-eters that are the measurable quantities and the formulationof matrix plane can be quantified. From Mie theory, oncescattering matrices have been derived, information aboutthe direction and polarization dependence of the scatteredlight can be extracted, so that absorption, scattering, andextinction cross sections for any arbitrary spherical particlewith dielectric function ε can be calculated. Since extinctedpower is the sum of the scattered and absorbed power, theabsorption cross section is simply σabs = σext − σsca, whilethe scattering and extinction cross sections can be calculatedfrom the total cross section of a spherical particle, that can bewritten as

σ = λ2m

2π

l=∞∑l=1

(2l + 1)[

Im{ϕEl

}+ Im

{ϕMl

}], (9)

where ϕE,Ml are the electric and magnetic scattering coeffi-

cients, respectively, and l is the orbital momentum number(e.g., l = 1 for a dipole). These coefficients admit analyticalexpressions in terms of spherical Bessel and Hankel functionsfor a homogeneous sphere [33]

ϕEl =

−εm jl(ρm)[

jl(ρ)

+ ρ j′l(ρ)]

+ ε[jl(ρm)

+ ρm j′l(ρm)]

jl(ρ)

εmhl(ρm)[

jl(ρ)

+ ρ j′l(ρ)]− ε

[hl(ρm)

+ ρmh′l

(ρm)]

jl(ρ)

ϕMl =

−ρ jl(ρm)j′l(ρ)

+ ρm j′l(ρm)jl(ρ)

ρhl(ρm)j′l(ρ)− ρmh

′l

(ρm)jl(ρ) ,

(10)

where ρ = (2πR/λ)√ε, ρm = (2πR/λ)

√εm, and the

prime represents the first differentiation with respect to theargument in parentheses. It should be noted that plasmonmodes are related to the poles of ϕE

l . In fact, the magneticcontribution to scattering produces only resonances forparticle sizes comparable to the wavelength. The multipolarpolarizability is proportional to ϕE

l , and in particular, thedipolar polarizability reads α = (3λ3/4π2)ϕE

l=1, where thedipolar electric scattering coefficient can be readily calculatedusing j1(x) = sin x/x2 − cos x/x and h1(x) = (1/x2 −i/x) exp(ix), so that (8) can be used instead of (9). The valueof α obtained is really accurate for describing gold spheres

with diameters up to 200 nm and simplified expression canbe found as, for example,

α = 3Vεm

× 1− 0.1(ε + εm)θ2/4

(ε + 2εm)/(ε − εm)− (0.1ε + εm)θ2/4− i(2/3)ε3/2m θ3

,

(11)

where V is the particle volume, ε and εm are the permittivitiesof the particle and the surrounding medium, and θ = 2πR/λis the size parameters that recover the electrostatic limit forθ = 0. Analogously, the dipolar polarizability of a coated goldnanosphere is given by

α = 3Vεm

× (R0/Ri)3(2εcoat + ε)(εcoat − εm)− (εcoat − ε)(2εcoat + εm)

(R0/Ri)3(2εcoat + ε)(εcoat + 2εm)− 2(εcoat − ε)(εcoat − εm)

,

(12)

where Ri is the internal radius of the core material describedby ε, the coating of permittivity εcoat extends up to a radiusR0, and the medium outside the particle has permittivityεm. An analogous formula can be derived for a bariumtitanate core particle with a gold nanoshell. Calculationsof the optical absorption and scattering efficiency of goldnanospheres and barium titanate-gold nanoshells will bepresented below. The required parameters for the code werethe value of the core and shell radii R1 and R2, the complexrefractive indices for the core, shell, and biomolecularcoating, and the surrounding medium nc, ns, nb, and nm,respectively, Figure 1.

The discrete-dipole approximation (DDA) has beenused as a complimentary method to Mie theory [36].DDA is a flexible and powerful technique for computingscattering and absorption by targets of arbitrary geometry.DDA calculations require choices for the locations and thepolarizabilities of the point dipoles that represent the targets.To approximate a certain geometry (e.g., a sphere or acore-shell-environment system, where the shell is made bygold) with a finite number of dipoles, we might considerusing some number of closely spaced, weaker dipoles inregions near the target boundaries to do a better job ofapproximating the boundary geometry. For a core-shellsystem, we use the following algorithm to generate the dipolearray. Given a coordinate reference system, (1) we generatea trial lattice defined by a lattice spacing d and coordinatesof the lattice point nearest the origin. (2) All lattice sites arelocated within the volume V of the core-shell-surroundingsystem (where the surrounding is considered as a sphereenveloping the core-shell system). (3) Try different values ofd and dipoles coordinates and maximize some goodness-of-fit criterion for a list of occupied sites i = 1, . . . ,N . Eachof these occupied sites represents a cubic subvolume d3 ofmaterial centered on the site. Particular efforts have beenaddressed to distinguish between lattice sites near the surfaceand those in the interior. (4) We therefore rescale the array


Surrounding medium

R1

nc

nsR2

nm

(a)

Surrounding medium

nm

R1nc

nsR2Coating

(b)

Figure 1: (a) Schematic sketch of a gold nanoshell, R1 and R2 are the core and shell radii, and the complex refractive indices for the core,shell, and the surrounding medium are nc, ns, and nm, respectively. (b) The same gold nanoshell with a biomolecular absorbed coating layer.Note that the shell radius is defined as the total radius minus the core radius.

by requiring that d = (V/N)1/3, so that the volume Nd3 ofthe occupied lattice sites is equal to the volume V of theoriginal target. (5) For each occupied N sites, assign a dipolepolarizability αi.

The assignation of dipole polarizabilities is a crucialquestion in a DDA method. Following the seminal paperof Draine and Flatau [37], the polarizability α(ω) can befound analytically in the long-wavelength limit |n|kd �1 as a series expansion in the powers of kd, with thecriterion that N > (4π/3)|n|3(kreff)3 where n is the refractiveindex and reff = (3V/4π)1/3 is the effective radius ofthe nanoparticles of volume V . As a consequence, formaterials with higher refractive indexes the DDA method canoverestimate absorption cross sections. A customized codefor the DDA calculations of gold nanoshells has been writtenadapting DDSCAT program that is a freely available code[38, 39].

The finiteness of the speed of light has important con-sequences that affect the response of gold nanoparticles.First of all, the electromagnetic field cannot penetratebeyond a certain depth inside the metal, the so-called skindepth, which is of the order of 15 nm in the vis-NIR.But more importantly, redshifts take place as the particlessize increases, and retardation effects play a significant rolewhen the diameter is a consistent fraction of the modewavelength λm in the surrounding medium, which is relatedto the free-space wavelength through λm = λ/

√εm. In

particular, opposite charges are separated by roughly oneparticle diameter in a dipole mode, so that the reaction ofone end of the particle to changes produced in the other endtakes place with a phase delay of the order of 4πR/λm, andconsequently, the period of one mode oscillation increasesto accommodate this delay. Analogously, quadrupoles andhigher-order modes produce more nodes in the distributionof the polarization charges induced on the surface of theparticle, which reduce the effective interaction distance.

Figure 2 shows the shift towards the NIR region for theefficiency of the absorption calculated using the Mie theoryand DDA approach for a gold nanoparticle (radius 120 nm,black line) and a barium titanate (BaTiO3)-gold nanoshell

5

4

3

2

1

0500 600 700 800 900 1000 1100

Abs

orpt

ion

(a.

u.)

Wavelength (nm)

Figure 2: Calculated spectra of the efficiency of absorption as afunction of the wavelength for gold nanoparticle (120 nm, blackline) and barium titanate-gold nanoshell (100 + 20 nm, red line).

(core 100 nm, gold 20 nm, red line). The barium titanaterefractive index was taken to follow the dispersion formulaas n2− 1 = 4.187λ2/(λ2− 0.2232). The refractive index of thesurrounding medium was considered to be nm = 1.34 + 0i atall wavelengths, close to the water.

Optical tunability plays a fundamental role for the iden-tification of size, shape, and core-shell composition of thenanoparticles for biomedical sensors and detection limitdefinition. In order to define the optical tunability, the redshifts must be carefully identified. In Figure 3, the tunabilityof the extinction cross section of a gold nanoparticle as afunction of the diameter size is reported, while in Figure 4,the same quantity for a core-gold nanoshell (core = BaTiO3,gold shell = 20 nm) is reported as a function of differentcore/gold ratios. In the case of a gold nanosphere, the totalextinction cross section increases linearly as the nanospheresize increases, with an initial value of σext (nm2) = 4871for a gold nanosphere diameter of 40 nm. Approximately,linear behavior of total extinction cross section is waited forcore/shells systems, Figure 4.


8

7

6

5

4

3

2

1

040 60 80 100 120 140 160

Nanosphere diameter (nm)

σ ext

(nm

2)

×104

Figure 3: Tunability of the extinction cross section of nanoparticles.Variation of the total extinction cross section as a function of thenanosphere diameter size. The extinction cross section changeslinearly with the gold nanosphere diameter giving the possibility ofdirect tuning of optical response with gold nanosphere dimension.

The increase in the extinction scattering with the na-noparticle volume has been related to increased radiativedamping in larger nanoparticles based on experimentalscattering spectra of gold nanospheres and core-gold systems[29–33]. This behavior suggests that larger nanoparticles ofhigh core/shell ratios would be more suitable for biologicalsensors applications based on light scattering. In the case ofnanoshells, the relative core-shell dimensions vary linearlythe magnitude of light extinction. This implies that adecrease in the core/shell ratio can be seen to be an effectivehandle in increasing the scattering contribution to the totalextinction. The next calculated quantity is the effect ofa biomolecular adsorbed layer on the gold surface for acore/shell system with fixed ratio (80 nm for BaTiO3 and20 nm for gold surface) on wavelength shifts Δλ for differentlayer dimensions. This quantity is particularly importantfor the definition of sensitivity response and FOM of abiosensor, as described by (3). In Figure 5, the progressiveplasmon resonance maximum wavelength red-shift is shownas a function of increased homogeneous layers of adsorbedbovine serum albumin (BSA, 170 mg/mol). The calculationwas made, as in the previous case, using (8) and (12),but including in the DDA approach progressive 1-to-3supplemental layers with a known refractive index [40]. Itis interesting to note that when increasing the size of thenanoparticle (from left black line to cyan line on right) anincreasing wavelength red-shift is observed. The reductionof the peaks is an artifact effect and the absorption quantityshould be normalized including the increasing nanoparticlevolume.

Figure 5 represents the most important result of thepresent paper. In fact, once the red-shift, Δλ, is accuratelyknown, it should be possible to quantify the refractive index

12000

11000

10000

9000

8000

7000

6000

5000

4000

3000

20000 1 2 3 4 5 6

Nanoshell core/shell ratio

σ ext

(nm

2)

Figure 4: Tunability of the extinction cross section as a functionof nanoshells core/shell ratios. As in the case of nanospheres, theextinction cross section shows a behavior close to be linear withcore/shell ratio increments (gold shell = 20 nm).

changes, γ, (7), to design high sensitive biosensors operatingat detection limit with effective optimal ∂n/∂c factor. Theabsolute magnitude of the optical cross section providesa limited reliable measure of the optical properties of apopulation of nanoparticles employed in the biomedicalsensing activity or other real-life biomedical applicationsbecause broad size nanoparticle population can have adifferent optical response with respect to few nanoparticlesbut with higher sizes. The red-shifts as a function ofnanoparticles diameter size, shown in Figure 5, are onequantity that can be measured in scanning near opticalmicroscopy (SNOM) technique, measurements that willprovide the next development of the present paper.

4. Conclusions

The development of biosensors devices requires sophisti-cated approaches to detect and to identify the analytes.Because the sensor signal-to-noise ratio increases withdecreasing size for many devices, many researchers areexpending considerable effort for miniaturizing sensingdevices down to nanoscale size. Gold nanoparticles have beenwidely used during the past few years in various technicaland biomedical applications. In particular, the resonanceoptical properties of nanometer-sized particles have beenemployed to design biochips and biosensors used as analyt-ical tools. The optical properties of nonfunctionalized goldnanoparticles and core-gold nanoshells play a crucial rolefor the design of biosensors where gold surface is used asa sensing component. Gold nanoparticles exhibit excellentoptical tunability at visible and near-infrared frequenciesleading to sharp peaks in their spectral extinction. In thispaper, we have simulated how the optical properties of goldnanoparticles are changed as a function of different sizes,shapes, and composition. We have shown that the optical


4

3

2

1

0600 700 800 900 1000 1100

Abs

orpt

ion

(a.

u.)

Wavelength (nm)

Figure 5: Plasmon resonance maximum red-shift for the absorp-tion in a core/shell nanoparticle (BaTiO3 80 nm, Au 40 nm)with a different number of coating layers of BSA biomolecules(170 mg/mol). Black line represents the uncoated nanoshell, redline represents the coating with a uniform BSA layer, the green linerepresents two layers and, finally, the cyan line shows the effect ofthe third layer.

tunability can be carefully tailored for particle sizes fallingin the range 100–150 nm. Nanoshells made by a bariumtitanate core have been considered and the optical responseof gold surfaces coated by 1-to-3 layer of BSA was taken inconsideration. The simulated results will help experimentaloptical tests that will be performed using nanoscale opticalscanning technology.

Acknowledgment

M. D’Acunto wishes to acknowledge the NanoICT Project foruseful support.

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