Electrical conductivity of cationized ferritin decorated gold nanoshellsRebecca Cortez, Joseph M. Slocik, Joseph E. Van Nostrand, Naomi J. Halas, and Rajesh R. Naik Citation: Journal of Applied Physics 111, 124311 (2012); doi: 10.1063/1.4729800 View online: http://dx.doi.org/10.1063/1.4729800 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/111/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Plasmon resonance based analysis of a single protein conjugated Au nanoshell Biointerphases 9, 031017 (2014); 10.1116/1.4895964 Surface plasmon resonances of protein-conjugated gold nanoparticles on graphitic substrates Appl. Phys. Lett. 103, 163702 (2013); 10.1063/1.4826514 Protein adsorption enhanced radio-frequency heating of silica nanoparticles Appl. Phys. Lett. 103, 043706 (2013); 10.1063/1.4816668 Interaction and diffusion of gold nanoparticles in bovine serum albumin solutions Appl. Phys. Lett. 102, 203705 (2013); 10.1063/1.4807672 Conductive atomic force microscopy study of single molecule electron transport through the Azurin-goldnanoparticle system Appl. Phys. Lett. 102, 203704 (2013); 10.1063/1.4807504

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Electrical conductivity of cationized ferritin decorated gold nanoshells

Rebecca Cortez,1,a) Joseph M. Slocik,2 Joseph E. Van Nostrand,3 Naomi J. Halas,4

and Rajesh R. Naik2

1Mechanical Engineering Department, Union College, Schenectady, New York 12308, USA2Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base,Ohio 45433, USA3Information Directorate, Air Force Research Laboratory, Rome, New York 13441, USA4Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA

(Received 17 March 2012; accepted 17 May 2012; published online 21 June 2012)

We report on a novel method of controlling the resistance of nanodimensional, gold-coated SiO2

nanoparticles by utilizing biomolecules chemisorbed to the nanoshell surface. Local electronic

transport properties of gold-coated nanoshells were measured using scanning conductance

microscopy. These results were compared to transport properties of identical gold nanoshells

biofunctionalized with cationized ferritin protein both with and without an iron oxide core

(apoferritin). Measured resistances were on the order of mega-ohms. White light irradiation effects

on transport properties were also explored. The results suggest that the light energy influences the

nanoshells’ conductivity. A mechanism for assembly of gold nanoshells with cationized ferritin or

cationized apoferritin is proposed to explain the resistivity dependence on irradiation. VC 2012American Institute of Physics. [http://dx.doi.org/10.1063/1.4729800]

I. INTRODUCTION

The concept of using molecules as memory elements or

switches in electronic devices1 has been an area of extensive

research. Nanoparticle biofunctionalization for use in nano-

dimensional electronic circuits and architectures, however,

has received considerably less attention.2 Extensive work in

molecular electronics and organic semiconductors has

revealed the importance of molecular structure and the mole-

cule’s local environment on molecular electronic transport

properties. Previous molecular electronics investigations uti-

lized a planar layer of molecules between electrodes, and

measured the effect of the ensemble on device transport

properties.3 Efforts to elucidate the local transport properties

of individual or small groups of molecules have typically

employed a nanodimensional gap between microelectronic

electrodes4–6 to make contact to the molecule, or an atomic

force microscope7 or scanning tunneling microscope8,9 probe

in direct contact with the molecule being explored. This is in

contrast to the exploration of local electronic transport prop-

erties of nanodimensional particles decorated with biological

molecules described in this article.

The techniques for nanoparticle biofunctionalization are

not new. For example, SiO2 nanoparticles bioconjugated with

antibodies for use in whole blood sol particle immunoassays

have existed for decades.10 Exciting developments in biofunc-

tionalized nanoparticles for delivery of medicines,11 imag-

ing,12,13 diagnosis,14 and disease therapy15 are revolutionizing

modern medicine. Previous efforts at nanoparticle biofunc-

tionalization in nanoelectronics have primarily focused on

techniques of directed self-assembly through bioconjugation

immobilization strategies utilizing antibody/antigen, aptamer/

target, or DNA complementary pairs.16,17 The objective in

these efforts was to achieve a “bottom-up” fabrication

approach to nanodimensional circuits and architectures, thus

avoiding costly nanometer resolution lithography techniques

employed in the “top-down” approach currently utilized by

the semiconductor manufacturing industry.

The concept of using biofunctionalized molecules to

obtain a desired electronic transport property with the added

benefit of achieving self directed, bottom-up nanodimen-

sional electronics fabrication would have great scientific

merit as well as technological benefit to the area of nanoelec-

tronics. One basic and nearly universal passive element in

electronic circuit architectures is the resistor. In this paper,

we explore a novel technique for controlling resistance at

nanodimensions using gold nanoshells biofunctionalized

with cationized ferritin. Ferritin is an intracellular spherical

protein cage important in nature for regulating and storing

iron in the form of an insoluble iron oxide nanoparticle.18

Consequently, it is highly valued in materials science as a

template for the confined synthesis of metal, metal sulfide,

and metal oxide nanoparticles; assembly with other inor-

ganic components; and as a catalyst for the growth of carbon

nanotubes.18 One advantage of this approach is that it ena-

bles nanoelectronic circuit fabrication requiring various con-

ductivity values at different locations to be constructed using

“off the shelf” nanoresistors fabricated using a facile synthe-

sis technique. Nanoparticle size control is possible by using

the well established Stober technique.19 We have explored

bare, unfunctionalized gold-coated SiO2 nanoshells as well

as those with biomolecules for use in directed self assembly.

Previous research using gold-coated silica has shown

that the nanoparticles are optically tunable for medical

applications across energies ranging from near ultraviolet to

mid-infrared wavelengths.13,20 Additional studies have also

indicated that texture21 and variations in the SiO2 core and

gold shell thickness22,23 affect the optical and plasmonic21,23a)Electronic mail: cortezr@union.edu.

0021-8979/2012/111(12)/124311/5/$30.00 VC 2012 American Institute of Physics111, 124311-1

JOURNAL OF APPLIED PHYSICS 111, 124311 (2012)

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properties of the nanoparticles. In this study, we also report

on the impact of white light energy on the local resistance of

both bare and biofunctionalized gold nanoshell particles sur-

rounding a silica core.

II. EXPERIMENT

Gold nanoshells were synthesized as described previ-

ously.24 The biomolecules used for these experiments con-

sisted of cationized ferritin from horse spleen with or without

an iron oxide core (cationized apoferritin). For nanoshell func-

tionalization, 2 ll of cationized ferritin (Sigma, 10 mg/ml)

was added to 500 ll of gold nanoshells (1.1� 1010 particles/

ml) in double deionized water and incubated for 4 h at room

temperature (Fig. 1). Unbound protein was removed through

three cycles of centrifugation at 450 rcf and resuspension of

functionalized nanoshells in 500 ll of double deionized water.

To obtain cationized apoferritin, we removed the ferrihydrite

iron oxide core from inside the protein cage by reductive dis-

solution with 0.5% mercaptopropionic acid (Sigma) in 0.1 M

acetate buffer pH 4.5 and repeated dialysis using 12 kDa

molecular weight cut off (MWCO) regenerated cellulose

Fisher brand dialysis tubing over 5 days. The removal of iron

oxide was confirmed by UV-Vis spectroscopy on a Varian

Cary 500 Scan UV-Vis-NIR spectrophotomer. The bare and

biofunctionalized nanoshells were dispersed on an n-type sili-

con substrate for subsequent characterization.

The bare and biofunctionalized gold nanoshells were

characterized by several microscopy techniques. Transmission

electron microscopy (TEM) measurements were performed

using a Phillips CM200 transmission electron microscope

operating at 200 kV. TEM samples were prepared by pipetting

10 ll of nanoshell solution onto a 3-mm-diameter copper grid

coated with carbon film (Electron Microscopy Sciences) and

air dried. Scanning electron microscopy (SEM) measurements

were performed using a Zeiss Supra 55 field emission electron

microscope.

Nanoparticle morphology and resistance were measured

using a Veeco Dimension V scanning probe microscope

(SPM). Morphology characterization was completed in tap-

ping mode using a Veeco RTESP probe, while local trans-

port measurements were completed in contact mode using a

Veeco MESP cobalt-chromium coated tip. Current-voltage

characterization (CVC) was completed either in the dark or

in the presence of white light (75 W halogen bulb source) ex-

posure to the nanoshells. The CVC response for the particles

resulted in a short-circuit, an open-circuit, or a stable

current-voltage response. Short- and open-circuit responses

are not shown and were believed to be due to tip-substrate

interactions and false engages, respectively. For the stable

CVC response, applied voltage biases ranged from 250 mV

up to several volts at which point the upper limit (near 1 lA)

was reached on the scanning conductance microscopy mod-

ule. At least a dozen particles were measured for the stable

CVC response for each of the bare and functionalized nano-

shell conditions in order to identify representative responses

for individual nanoparticles. The tungsten halogen light

source operated at a color temperature of 3000 K, covering

the visible light range from 400 nm to 700 nm, and was piped

into the SPM chamber via a fiber optic cable. A schematic of

the CVC measurement set-up within the SPM is provided in

Fig. 2.

III. RESULTS AND DISCUSSION

The gold-coated silica nanoparticles decorated with cat-

ionized ferritin utilized in this effort can be seen in the TEM

micrograph and schematic shown in Fig. 3. Note the pres-

ence of the cationized ferritin particles populating the surface

of the gold coated SiO2 particles (Fig. 3(a)). A schematic of

the protein decorated nanoshell is shown in Fig. 3(b). SEM

imaging revealed a typical nanoparticle diameter of 160 nm.

The gold nanoshells’ morphology as measured by atomic

force microscopy (AFM) is shown in Fig. 4. The height (Fig.

4(a)) and phase (Fig. 4(b)) images reveal both the morphol-

ogy and the texture present on the nanoshells, as well as the

monodisperse nanoshell distribution. The morphologies of

the bare and biofunctionalized nanoshells were similar. The

average and median Ra surface roughness values (ten par-

ticles each) of the bare and biofunctionalized nanoshells are

shown in Table I. The decrease in the median Ra surface

FIG. 1. Assembly of Au nanoshells with cationized ferritin or cationized

apoferritin.

FIG. 2. Schematic of the electrical measurement set-up within the SPM

system.

124311-2 Cortez et al. J. Appl. Phys. 111, 124311 (2012)

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roughness on the biofunctionalized compared to the bare

nanoshells suggests that the biomolecules influence the nano-

shell surface texture.

The bare and biofunctionalized nanoshells were exposed

to either no light or white light as CVC measurements25

were made. Typical CVC responses for each of the nanoshell

coatings examined are shown in Fig. 5. The corresponding

resistance values for the bare and cationized ferritin deco-

rated gold nanoshells are provided in Table II. Fig. 5(a) illus-

trates the current-voltage response of the bare nanoshells.

Comparison of the gold nanoshells’ CVC response with

those of the biofuntionalized nanoshells shown in Figs. 5(b)

and 5(c) clearly indicates that the biomolecule coatings alter

FIG. 4. AFM images showing the morphology of gold-coated SiO2 nanopar-

ticles. (a) Height scan; the black to white grayscale is 300 nm; and (b) the

corresponding phase image where the black to white grayscale is 45�. Both

images are 2 lm by 2 lm.

TABLE I. Measured surface roughness.

Ra

Au nanoshells

(NS) (nm)

NS biofunctionalized

with cationized

ferritin (nm)

NS biofunctionalized

with cationized

apoferritin (nm)

Average 7.96 6 0.92 5.25 6 1.54 6.47 6 0.67

Median 8.41 4.97 6.61

FIG. 5. Local transport properties of gold-coated SiO2 nanoparticles. (a)

Bare gold nanoshells; (b) cationized ferritin coated gold nanoshells; and (c)

cationized ferritin (no iron oxide core) decorated gold nanoshells.

FIG. 3. (a) TEM image of Au nanoshells decorated with cationized ferritin.

Arrows point to iron oxide ferrihydrite cores of ferritin; (b) schematic repre-

sentation of cationized ferritin decorated gold nanoshell.

124311-3 Cortez et al. J. Appl. Phys. 111, 124311 (2012)

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the gold nanoshells’ electrical response. The resistance mag-

nitude for the gold nanoshell and cationized ferritin deco-

rated gold nanoshell nanoparticles was measured to be on

the order of MX. To the authors’ knowledge, no direct mea-

surement of gold nanoshell resistance has been reported in

the literature. Earlier research characterized the electrical

response of thin film device structures utilizing ensembles of

gold-coated silica nanoparticles; measured device resistances

of 100X were reported.4 The resistances reported herein

were measured directly across individual nanoparticles. The

large magnitude of resistance may be due to impurities in the

gold coating or high contact resistance due to the shells’

nanodimensions. For example, this may be due to the spheri-

cal nature of the nanoshells and the limited contact area

between the nanoshells and the silicon substrate.

The cationized apoferritin decorated nanoshells did not

exhibit ohmic behavior thereby prohibiting the determination

of their resistance. As shown in Fig. 5(c), these nanoparticles

behaved more like diodes as at least 0.75 V bias was neces-

sary to elicit an electrical response from the material.

The cationized ferritin decreased the gold nanoshells’

conductivity in the absence of light as indicated in Table II.

The impact of white light energy was significant for the cat-

ionized ferritin and cationized apoferritin nanoparticles as

shown in Figs. 5(b) and 5(c). Light increased the cationized

ferritin particles’ conductivity by a factor of four. Although

light exposure did reduce the resistance of the bare gold

nanoshells and the cationized ferritin coated nanoshells, it

should be noted that the cationized apoferritin’s resistance

increased in the presence of light. The diode behavior

observed with light was also present in the cationized apofer-

ritin coated nanoshells. This suppression of the conductivity

of the cationized apoferritin sample is clearly seen in Fig.

5(c). Earlier work suggested that variations in the plasmonic

response of gold nanoshells could be controlled by the nano-

shells’ surface roughness.21 It may be possible that the pro-

tein structure differences and/or the nanoparticles’ surface

roughness variations in the present study were responsible

for the impact of light on the measured resistances.

An additional explanation for the impact of light on the

conductivity of the biofunctionalized nanoshells is believed

to originate from the dynamic protein structure of ferritin.

As indicated, if the nanoshell has no iron or iron oxide on

it, it is empty and exists as a protein cage. It may be possi-

ble that without the iron oxide core that the nanoshells are

getting hot from irradiation which causes the empty protein

cage to unfold and denature. This light induced swelling/

denaturation of the protein may have caused a change in the

individual protein-nanoshell interfacial distance thus affect-

ing the resistivity. By comparison, with the iron oxide core

(cationized ferritin) when the nanoshells become heated by

light, the protein oxide complex has a higher thermal stabil-

ity and hence retains its quaternary structure. To better

understand whether the silicon sample was heating up while

exposed to the white light, a type K thermocouple was

placed directly on a silicon wafer, and its surface tempera-

ture was measured as the sample was exposed to the white

light. After 3 h time, the maximum increase in sample sur-

face temperature was 1.4 �C. Based on this measured tem-

perature rise coupled with the assumption that the

nanoshells dispersed on the silicon wafer were exposed to a

similar temperature, it was concluded that the light energy

combined with the protein behavior was responsible for the

differences in the electrical behavior of the nanoshells,

rather than bulk heating of the substrate.

IV. CONCLUSIONS

In conclusion, we have demonstrated a facile technique

for nanodimensional resistive circuit elements synthesis uti-

lizing gold nanoshells biofunctionalization with biomole-

cules. We believe this to be the first demonstration of local

measurement and resistivity control of a nanoparticle at the

nanoscale, and provides a methodical approach to achieving

the desired resistance for a nanoelectronic circuit by utilizing

conducting nanoparticles biofunctionalized with an appropri-

ate protein sequence. Also, we envision potential gains in

transport properties of the bioconjugated nanoshells by sub-

stituting the iron oxide core with alternate metal or quantum

dot nanoparticles in ferritin.

ACKNOWLEDGMENTS

The authors would like to thank Felica Tam of Rice

University for her assistance in synthesizing the gold nano-

shells. This material is based upon work supported by the

National Science Foundation under Grant Nos. 0820032

and 0824341.

1A. Aviram and M. Ratner, Chem. Phys. Lett. 29, 277 (1974).2R. Andres, J. Bielefeld, J. Henderson, D. Janes, V. Kolagunta, C. Kubiak,

W. Mahoney, and R. Osifchin, Science 273, 1690 (1996).3T. He, D. Corley, M. Lu, N. D. Spigna, J. He, D. Nackashi, P. Franzon,

and J. Tour, J. Am. Chem. Soc. 131, 10023 (2009).4M. Islam, R. Geiger, and J. Kim, Nanotechnology 18, 255203 (2007).5J. Park, A. Pasupathy, J. Goldsmith, C. Chang, Y. Yalsh, J. Petta, M. Rin-

koski, J. Sethna, H. Abruna, P. McEuen et al., Nature 417, 722 (2002).6A. Bezryadin, C. Dekker, and G. Schmidt, Appl. Phys. Lett. 71, 1273

(1997).7C. Cen, S. Thiel, G. Hammerl, C. Schneider, K. Andersen, C. Hellberg, J.

Mannhart, and J. Levy, Nature Mater. 7, 298 (2008).8F. Mohn, J. Repp, L. Gross, G. Meyer, M. Dyer, and M. Persson, Phys.

Rev. Lett. 105, 266102 (2010).9X. Cui, A. Primak, X. Zarate, J. Tomfohr, O. Sankey, A. Moore, T. Moore,

D. Gust, G. Harris, and S. Lindsay, Science 294, 571 (2001).10J. Leuvering, P. Thal, M. van der Maart, and A. Schuurs, J. Immunoassay

1, 77 (1980).11G. Paciotti, L. Myer, D. Weinreich, D. Goia, N. Pavel, R. McLaughlin,

and L. Tamarkin, Drug Delivery 11, 169 (2004).12Y. Wang, X. Xie, X. Wang, G. Ku, K. Gill, D. O’Neal, G. Stoica, and L.

Wang, Nano Lett. 4, 1689 (2004).13C. Loo, A. Lowery, N. Halas, J. West, and R. Drezek, Nano Lett. 5, 709

(2005).14J. Slocik, F. Tam, N. Halas, and R. Naik, Nano Lett. 7, 1054 (2007).15L. Hirsch, R. Stafford, J. Bankson, S. Sershen, B. Rivera, R. Price, J. Hazle,

N. Halas, and J. West, Proc. Natl. Acad. Sci. U.S.A. 100, 13549 (2003).

TABLE II. Measured resistivity (average).

Illumination

Au nanoshells

(NS) (MX)

NS biofunctionalized

with cationized ferritin (MX)

None 1.56 6 0.06 3.77 6 0.18

White light 1.18 6 0.04 0.90 6 0.03

124311-4 Cortez et al. J. Appl. Phys. 111, 124311 (2012)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

202.28.191.34 On: Mon, 22 Dec 2014 07:40:10

16M. Grzelczak, J. Vermant, E. Furst, and L. Liz-Marzan, ACS Nano 4,

3591 (2010).17N. Cady, M. Bergkvist, N. Fahrenkopf, P. Rice, and J. Van Nostrand, in

Proceedings of 2010 IEEE International Symposium on Circuits and Sys-tems (ISCAS), (2010), p. 1959.

18U. Masaki, M. Klem, M. Allen, P. Suci, M. Flenniken, E. Gillitzer, Z. Varp-

ness, L. Liepold, M. Young, and T. Douglas, Adv. Mater. 19, 1025 (2007).19W. Stober, A. Fink, and E. Bohn, J. Colloid Interface Sci. 26, 62 (1968).20A. Lin, N. Lewinski, J. West, N. Halas, and R. Drezek, J. Biomed. Opt. 10,

064035 (2005).

21H. Wang, G. Goodrich, F. Tam, C. Oubre, P. Nordlander, and N. Halas,

J. Phys. Chem. B Lett. 109, 11083 (2005).22S. Oldenburg, J. Jackson, S. Westcott, and N. Halas, App. Phys. Lett. 75,

2897 (1999).23B. Brinson, J. Lassiter, C. Levin, R. Bardhan, N. Mirin, and N. Halas,

Langmuir 24, 14171 (2008).24S. Oldenburg, S. Wescott, R. Averitt, and N. Halas, J. Chem. Phys. 111,

4729 (1999).25J. Van Nostrand, R. Cortez, Z. Rice, N. Cady, and M. Bergkvist, Nano-

technology 21, 415602 (2010).

124311-5 Cortez et al. J. Appl. Phys. 111, 124311 (2012)

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