Aqueous synthesis of ZnTe/dendrimer nanocomposites and their antimicrobial activity: implications in therapeutics

PAPER www.rsc.org/nanoscale | Nanoscale

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Aqueous synthesis of ZnTe/dendrimer nanocomposites and their antimicrobialactivity: implications in therapeutics†

S. Ghosh,a D. Ghosh,a P. K. Bag,b S. C. Bhattacharyac and A. Saha*a

Received 18th August 2010, Accepted 15th October 2010

DOI: 10.1039/c0nr00610f

The present strategy proposes a simple and single step aqueous route for synthesizing stable, fluorescent

ZnTe/dendrimer nanocomposites with varying dendrimer terminal groups. In these hybrid materials,

the fluorescence of the semiconductor combines with the biomimetic properties of the dendrimer

making them suitable for various biomedical applications. The ZnTe nanocomposites thus obtained

demonstrate bactericidal activity against enteropathogenic bacteria without having toxic effects on the

human erythrocytes. The average size of the ZnTe nanoparticles within the dendrimer matrix was in the

range of 2.9–6.0 nm, and they have a good degree of crystallinity with a hexagonal crystal phase. The

antibacterial activities of the ZnTe/dendrimer nanocomposites (ZnTe DNCs) as well other

semiconductor nanocomposites were evaluated against enteropathogenic bacteria including multi-drug

resistant Vibrio cholerae serogroup O1 and enterotoxigenic Escherichia coli (ETEC). ZnTe DNCs had

significant antibacterial activity against strains of V. cholerae and ETEC with minimum inhibitory

concentrations ranging from 64 to 512 mg ml�1 and minimum bactericidal concentrations ranging from

128 to 1000 mg ml�1. Thus, the observed results suggest that these water-soluble active nanocomposites

have potential for the treatment of enteric diseases like diarrhoea and cholera.

1. Introduction

Hybrid dendrimer nanocomposites (DNCs) are an emerging

class of new materials that hold significant promise in diverse

fields, such as bio-imaging, non-linear optics, sensors, catalysis

and cancer treatment, and can serve as building blocks for highly

ordered nanostructures including self-assembled ultrathin

multilayers and smart nano-devices.1–6 Dendrimer-mediated

synthesis exhibits a greater degree of control with respect to their

composition, size, shape and surface functionalities, which in

turn, imparts stability, biocompatibility and water-solubility to

the nanocomposites. So, these novel hybrid materials are ideally

suited for various biomedical applications.7–12

There have been several reports on the synthesis of metal

sulfides, like CdS and ZnS nanocrystals (quantum dots), in

a dendrimer matrix.10–16 We earlier observed that sulfide based

semiconductor nanocrystals synthesized in a dendrimer matrix

showed relatively low luminescence quantum efficiency

compared to telluride based ones. However, literature on the

synthesis of metal telluride nanocrystals in a dendrimer matrix is

limited.17 In the present study, we have endeavoured to synthe-

size water soluble, biologically suitable ZnTe/dendrimer nano-

composites because of the exciting optical properties of ZnTe

nanocrystals (direct transition band gap of 2.26 eV at room

aUGC-DAE Consortium for Scientific Research, Kolkata Centre, III/LB-8Bidhannagar, Kolkata, 700 098, India. E-mail: [email protected];Fax: +91 33 2335 7008; Tel: +91 33 2335 6541bDepartment of Biochemistry, University of Calcutta, 35 BallygaungeCircular Road, Kolkata, 700 019, IndiacDepartment of Chemistry, Jadavpur University, Kolkata, 700 032, India

† Electronic supplementary information (ESI) available: Dynamic lightscattering, atomic force microscopy and hemolytic activity of thenanocomposites. See DOI: 10.1039/c0nr00610f

This journal is ª The Royal Society of Chemistry 2011

temperature).18 Various synthetic methodologies have been tried

for the production of colloidal ZnTe nanocrystals.19 Among

these, the most successful approach for obtaining good quality

particles was the pyrolysis of organometallic precursors at high

temperature.20 However, nanoparticles obtained via organome-

tallic routes are not fully compliant, or rather fail the criteria for

biomedical applications.21–25 Hence, aqueous synthesis is an

alternative and important strategy to directly prepare water-

dispersed quantum dots (QDs) from the point of view of bio-

logical applications. However, ZnTe nanocrystals synthesized

earlier in aqueous medium were quite large and also of poor

luminescent qualities.26 Here, we have demonstrated that ZnTe/

dendrimer nanocomposites with a narrow size distribution and

reasonably good quantum yield can be synthesized through an

aqueous route and are useful for biological applications. The

dendrimer prevents agglomeration and stabilizes the nano-

particles, making it possible to tune solubility. Furthermore,

dendrimer provides a means of immobilization of the nano-

particles on a solid support and to afford controllable self-

assembly of entrained nanoclusters on a variety of surfaces

through chemi-sorption.13,16 Additionally, the surface group of

the dendrimer remains free and can be utilized for conjugation

with other biomolecules for biosensors and biolabelling experi-

ments.11,13–16 Formation of nanoparticles in a dendrimer matrix

opens up the possibility of fabricating new materials and devices

with novel or enhanced physical and chemical properties as

interactions between proximal nanocrystals give rise to new

collective phenomena.

In recent times, inorganic nanoparticles with antimicrobial

activity have been emerging as a new class of biomedical mate-

rials to fulfill the increasing general demands for hygiene in daily

life due to their large specific surface area and high bioactivity.27

A number of nanoparticles with antimicrobial activities have

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been reported recently, particularly, silver or gold nanoparticles

have been used extensively in many bactericidal fields.28–30 Their

predominant antimicrobial activity can be attributed to the

strong cytotoxicity to various bacterial cells, i.e., they can

interact with the functional groups on the bacterial cell surface

and inactivate bacteria.31–34 QDs have superior size-dependent

optical properties and play an important role in the studies of

complex microbial populations and the identification of bacteria,

through the construction of probe-conjugated QDs for single

bacterium imaging.35,36 In contrast, little attention has been paid

to exploring the potential of antimicrobial activities of QDs.37–39

The incidence of diarrhoeal disease varies largely according to

geography, with an estimated incidence between 1.3 and 2.3

episodes of diarrhoea per child per year in developed countries

compared with 3–9 episodes per child per year in the developing

world. Diarrhoea still accounts for 1.6–2.5 million deaths

annually in children below 5 years of age.40 Among the diar-

rhoeal diseases, cholera is a serious epidemic disease caused by

the Gram-negative bacterium Vibrio cholerae.41 Treatment with

oral rehydration solution (ORS) has reduced the levels of

mortality in children and adults by dehydration, but not

morbidity for diarrhoea.42 Although the drugs such as race-

cadotril and loperamide are used to treat secretory diarrhoea,

these drugs have side effects such as bronchospasm, vomiting

and fever.43 Antibiotics are used to treat diarrhoeal patients.

However, antibiotic resistance has been of great concern due to

the extensive use of classical antibiotics.44,45 Antibiotic resistance

among the pathogens is a serious clinical problem in the treat-

ment and containment of the disease. Different inorganic

nanoparticles have elicited much interest due to their potential

for achieving a specific process and selectivity in biological and

pharmaceutical applications.32–34,46 However, all studies are

focused on the antimicrobial effects of cadmium based particles

such as CdSe and CdTe-core QDs. Because their chemical and

physical properties play vital roles in their bioactivities, QDs in

a polymer matrix may have a distinct interaction with microbes.

The antibacterial properties of semiconductor/nanocomposites

have not been studied yet. Thus, it is of great importance to

investigate the antimicrobial activities of semiconductor den-

drimer nanocomposites that are composed of other zinc based

core materials. The present investigation proposes an aqueous

synthesis of ZnTe/dendrimer nanocomposites, which show

significant antimicrobial activity towards enteropathogens

without having toxicity on human erythrocytes.

2. Experimental

2.1 Reagents

Starburst PAMAM dendrimers of generation having surface

amino group, half generation 3.5 with carboxyl end groups,

hydroxyl end groups (G4) and with succinamic acid end groups

were purchased from Sigma Aldrich. ZnSO4$7H2O,

CdCl2$4H2O, sodium thiosulfate (Na2S2O3$5H2O), were

obtained from Merck, India, and Telluric acid (H2TeO4$2H2O)

and Sodium borohydride (NaBH4) (from BDH, India). All

chemicals used were of analytical reagent grade. Milli-Q water

(Millipore) and methanol (HPLC grade) were used as solvents.

The dendrimer solution (1.73 � 10�4 M) in water under N2

1140 | Nanoscale, 2011, 3, 1139–1148

atmosphere at 10 �C was prepared freshly before the synthesis

taking into account the manufacturer’s value of the dendrimer

weight fractions in methanol and the known dendrimer densities.

2.2 Preparation of NaHTe solution

In the preparation of NaHTe solution, 4.5 mg of telluric acid was

dissolved in minimum amount of water (�0.3 ml) and the solu-

tion was heated with borohydride under a continuous flow of

nitrogen gas until the colour changed from black to colourless.

Then the colourless solution was kept in an ice bath for 2–3 h

after which a white precipitate of sodium tetraborate came out.

The clear supernatant containing NaHTe free from contamina-

tion of borates was used for the synthesis.

2.3 Preparation of ZnTe/dendrimer nanocomposites

A typical preparation of ZnTe/dendrimer nanocomposites with

an initial Zn2+/Te2� molar ratio of 1 : 1 was as follows: 5 ml

aliquot of Zn2+ stock solution (2.0 mM) in water was added to

10 ml of dendrimer solution at 10 �C and vigorously stirred for

2 min to make the final concentration of Zn2+ and dendrimer as

2 � 10�3 M and 1 � 10�4 M, respectively. Then, NaHTe solution

(as prepared above) was taken in a syringe and injected to the

argon purged solution mixture of Zn2+ and dendrimer. The

resulting solution was pale yellow and did not show any evidence

of precipitates. In a similar manner, CdTe/dendrimer nano-

composites has been prepared using Cd2+ stock solution in place

of Zn2+ stock solution at the same concentration.17 The ZnS/

dendrimer and CdS/dendrimer nanocomposites used in the

microbial study were prepared following our methods reported

earlier.15,47 It was observed that there was no significant change in

absorption spectral profile of the semiconductor nanoparticles in

dendrimer matrix for a period of one month. Unaltered

absorption onset suggests that the particle size has not changed.

The as-prepared nanocomposite solutions were concentrated 4–5

times by freeze-drying, in which a pressure of 10 mTorr and

temperature of �85 �C were maintained. Further, on gradual

addition of a ‘non-solvent’ (isopropanol) to the colloidal solution

of nanocomposites, different fractions precipitated. The precip-

itates thus obtained were washed with acetone followed by

diethyl ether to remove residual chemicals. The nanocomposites

were then dried and stored in vacuum desiccators.

2.4 Characterization and spectral analyses

Transmission electron microscopy (TEM) was carried out on

JEOL-2010 with acceleration voltage of 200 kV. A drop of as-

prepared solution of ZnTe/dendrimer nanocomposite was placed

on a carbon-coated copper grid of 300 mesh and dried before

putting it on to the TEM sample chamber. About eight images

are taken for each sample. The crystal phases of the nanocrystals

were characterized by X-ray powder diffraction (XRD)

measurements using a Philips Analytical X-Ray B.V. diffrac-

tometer type PW 1710 equipped with graphite mono chromat-

ized Cu-Ka radiation (l ¼ 1.54056 �A). A scanning rate of 0.02�

per 2 s in 2q range from 10�–80� was employed.

UV–Vis absorption spectra were recorded with a Shimadzu

UV-1601PC spectrophotometer. Photoluminescence measure-

ments of ZnTe NPs were monitored by Perkin Elmer LS-55

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luminescence spectrometer. pH measurement was performed on

a Jenways ion-meter. Size distribution and zeta potential of

ZnTe/dendrimer nanocomposites were determined by dynamic

light scattering spectrophotometer (Model DLS - nanoZS,

Zetasizer, Nanoseries, Malvern Instruments). Samples were

filtered several times through a 0.22 mm Millipore membrane

filter prior to recording measurements. The zeta potential was

calculated from the electrophoretic mobility using the Smo-

luchowski equation with the help of commercial software. The

results are expressed as mean values of three samples. Zeta

potential measurements were performed at 25 �C. The FTIR

spectra were recorded with Perkin Elmer, Spectrum GX equip-

ment with a resolution of 2 cm�1 and scan range of 1000–4000

cm�1. Fluorescence images were recorded using fluorescence and

phase-contrast microscope (Model BX51/B52; Olympus, Japan).

2.5 Screening for antimicrobial activity

The antibacterial activity of the nanocomposite against some of

the pathogenic Gram-positive and Gram-negative bacteria was

determined by agar-diffusion assay as described previously48 and

following the method of Reeves et al.49 Among the Gram-nega-

tive bacteria, clinical isolates of Vibrio cholerae strains NB2 (O1

serotype); and Escherichia coli strain PC80 (Enterotoxigenic

Escherichia coli, ETEC) were used in this study. The other strains

used in this study were Gram-positive bacteria including Staph-

ylococcus aureus ATCC 25923, and Bacillus subtilis ATCC 6623.

Bacterial strains were first grown in Mueller-Hinton broth

(MHB) (HiMedia, Mumbai, India) under shaking condition for

4 h at 37 �C, and after the incubation, 1 ml of culture was spread

on Mueller-Hinton agar (MHA) (HiMedia). The wells were

made using a sterile 6 mm cork borer in the inoculated MHA

plate. The wells were filled with 50 ml (2 mg ml�1) of the samples

of nanocomposites (re-suspended in water) and blanks (water).

Tetracycline (10 mg/50 ml) was used as antibacterial positive

control and E. coli ATCC 25922 was included for quality

assurance. Zone diameter was measured after 24 h incubation at

37 �C. The photograph was taken in Gel documentation system

(Vilber Lourmat, France).

2.6 Determination of minimum inhibitory concentration (MIC)

and minimum bactericidal concentration (MBC)

MIC and MBC of the dendrimer nanocomposites were deter-

mined using broth micro dilution as described previously48

following the method recommended by the National Committee

for Clinical Laboratory Standards.50,51 An inoculum of the

microorganism was prepared from 24 h MHB cultures and

suspensions were adjusted with turbidity equivalent to that of

a 0.5 McFarland standard. Bacterial suspensions were further

diluted 1 : 10 in sterile MHB to obtain a final inoculum of 5� 105

CFU ml�1 (colony forming units). The 96-well round bottom

sterile plates were prepared by dispensing 180 ml of the inoculated

broth into each well. A 20 ml aliquot of the sample was added.

The concentrations of samples tested were 1, 2, 4, 8, 16, 32, 64,

128, 500, and 1000 mg ml�1. Dilutions of tetracycline served as

positive control, while broth with 20 ml of water was used as

negative control. E. coli ATCC 25922 was included for quality

assurance purposes. Plates were covered and incubated for 24 h

This journal is ª The Royal Society of Chemistry 2011

in ambient air at 37 �C. After incubation, minimum inhibitory

concentrations (MIC) were read visually; all wells were plated to

nutrient agar (Hi-Media) and incubated. The minimal bacteri-

cidal concentration (MBC) was defined as a 99.9% reduction in

CFU from the starting inoculum after 24 h incubation interval.

2.7 The time-kill kinetic study

Cultures of V. cholerae NB2 in MHB (around 1� 107 CFU ml�1)

were incubated separately in the absence (control) and in the

presence of ZnTe_G4.NH2 at a concentration of 128 mg ml�1

(MBC) for a period of 12 h at 37 �C. Samples of the bacterial

cultures were removed at 2 h intervals to record survival counts,

expressed as CFU ml�1. The surviving log10 CFU ml�1 was

plotted against time.

2.8 Haemolytic activity

A haemolysis test was employed to determine cellular toxicity of

the nanocomposites as previously described.52 The nano-

composites at concentrations ranging from 1� to 8�MBC, were

incubated with an equal volume of 1% human red blood cells in

phosphate buffered saline (10 mM PBS, pH 7.4) at 37 �C for 1 h.

Non-haemolytic and 100% haemolytic controls were the buffer

alone and the buffer containing 1% Triton X-100, respectively.

Cell lysis was monitored by measuring the release of haemoglo-

bin at 540 nm.

2.9 Statistical analysis

Values are expressed, as mean � S.D. Statistical significance was

determined using Student’s t-test. Values with p < 0.05 were

considered significant.

3. Results and discussion

3.1 Characterization of particle size by TEM and XRD

A typical transmission electron microscopic image for ZnTe/

dendrimer nanocomposites is shown in Fig. 1a, which yields an

average particle size of about 3.0 nm for the synthesis in water at

10 �C using NH2 terminated dendrimer. The shape of the ZnTe

nanocrystals are not spherical but slightly oblate as evident from

the TEM image (Fig. 1a). The corresponding particle size of

ZnTe NPs as determined from the respective absorption onset is

2.9 nm. This is in good agreement with the size calculated from

TEM data. Lattice fringes can be discerned in the high resolution

TEM image, which suggest good crystallinity of ZnTe NPs. The

inter-planar distance was found to be 3.9 �A. The SAED pattern

displays bright rings at a distance of 3.11, 2.24, 1.93, 1.33 and

1.167 �A corresponding to 003, 103, 111, 210, and 300 lattice

planes of the hexagonal crystal phase of ZnTe (JCPDS no. 83–

0967). It appears that the size of the ZnTe nanocrystal in the NH2

terminated dendrimer is smaller and more monodisperse than

that of ZnTe nanocystal (3.3 nm and 6.0 nm) using COOH and

OH terminated dendrimer respectively (as shown in Fig. 1b, c).

ZnTe nanocrystals in the NH2 terminated dendrimer matrix are

less aggregated compared to the COOH and OH terminated

dendrimer (ESI†), presumably due to the existence of the

primary as well as internal secondary amine groups of

Nanoscale, 2011, 3, 1139–1148 | 1141

Fig. 1 (a) TEM image of ZnTe/dendrimer nanocomposites using NH2

terminated dendrimer. (b) TEM image of ZnTe/dendrimer nano-

composites using COOH terminated dendrimer. (c) TEM image of ZnTe/

dendrimer nanocomposites using OH terminated dendrimer.

Fig. 2 XRD pattern of ZnTe/dendrimer nanocomposites.

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dendrimers that more effectively prevent the aggregation of

ZnTe NPs during their formation process. It may be noted that in

our earlier study, the size of the particles of ZnS in the dendrimer

matrix could be tuned in the range of 2.2–3.1 nm whereas, in case

1142 | Nanoscale, 2011, 3, 1139–1148

of CdS, it was within 2.8–3.9 nm. However, the observed size

tunability in the presently synthesized ZnTe nanocrystals is in the

range of 2.9–6.0 nm.

Typical X-ray powder diffraction (XRD) patterns of the as-

prepared ZnTe nanocrystals are shown in Fig. 2. The three

distinct diffraction peaks were observed at 2q values of 27.6�,

38.4� and 48.4�, respectively, corresponding to the (101), (103)

and (112) crystalline planes of hexagonal ZnTe (JCPDS No. 830–

967). The broad nature of the XRD peaks could be attributed to

the nano-crystalline nature of ZnTe particles. No other charac-

teristic peaks of impurities were observed.

3.2 Nanoparticles/PAMAM dendrimer binding

It was found that a strong coordinating interaction did exist

between the nanoparticles and the amide moieties—tertiary

amine groups and surface end groups such as amino, carboxyl,

hydroxyl, and sucinamic acid groups of PAMAM-dendrimer.

Typical FTIR spectra of dendrimer and ZnTe/dendrimer nano-

composites with terminal COOH are depicted in Fig. 3. It

appears that the bands at 1562 and 1641 cm�1 assigned for

symmetric stretching mode of C–O or bending mode of O–H are

shifted to 1567 and 1649 cm�1, respectively. Again, the band at

3286 cm�1 corresponding to stretching mode of hydroxyl group

(OH) is shifted to 3357 cm�1 suggesting that NPs are attached to

the dendrimer through the surface carboxyl (COOH) group.

3.3 Functionalization of ZnTe/dendrimer nanocomposites

We have previously reported that subtle variation of the terminal

group of poly(amidoamine) (PAMAM) dendrimers can effec-

tively control the quality of nanoparticles formed.15,47 In this

study, we describe the templated synthesis and characterization

of ZnTe/dendrimer nanocomposite using PAMAM dendrimer

with different terminal groups such as amino (NH2), carboxyl

(COOH), hydroxyl (OH) and succinamic acid (SAH). These

nanocomposites were prepared under identical conditions (at

10 �C, fixed molar ratio) following the method described earlier.

Fig. 4 displays the optical spectra of ZnTe/dendrimer nano-

composites. It was observed that a sharp excitonic peak was

This journal is ª The Royal Society of Chemistry 2011

Fig. 3 FTIR spectra of COOH terminated dendrimer and ZnTe/den-

drimer nanocomposites.

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obtained in the case of ZnTe/dendrimer nanocomposites with

terminal NH2 (ZnTe_G4.NH2) and COOH (ZnTe_G3.5COOH),

whereas a relatively broad spectrum and a broad shoulder were

observed with terminal OH (ZnTe_G4.OH) and SAH

(ZnTe_G3.5SAH) respectively. ZnTe nanocrystals in the den-

drimer matrix (in the case of amine and carboxyl end groups of

dendrimer) show well-defined 1s–1s electronic transitions in the

absorption spectra. The absorbance edge of the nanocrystals was

329 and 338 nm respectively and the band gap was calculated to

be 3.7 and 3.6 eV. The absorbance edge blue-shifts 219 nm

compared with bulk ZnTe (the band gap is 2.26 eV, and the

absorbance edge is 548 nm). In fact, the value of the energy gap,

estimated from Fig. 4, reflects a considerable blue shift relative to

Fig. 4 Functionalization of ZnTe/dendrimer nanocomposites in the presen

(SAH) terminated dendrimers.

This journal is ª The Royal Society of Chemistry 2011

the absorption band edge of bulk ZnTe. Generally, the wave-

length of the exciton absorption band decreases with decreasing

particle size as a result of quantum confinement of the photo-

generated electron hole pairs.53

On the other hand, NH2- and COOH-terminated dendrimers

produced smaller ZnTe NPs (2.9 and 3.5 nm, respectively) as

compared to SAH-terminated dendrimer (5.0 nm). This is

probably due to the differential interactions of ZnTe NPs with

the surface groups of the dendrimers. Strong electrostatic inter-

action between Zn2+ and NH2 or COOH groups controls the

growth and stabilization of the ZnTe NPs within the dendrimer

matrix. Similar terminal group dependence was observed in ZnS/

dendrimer nanocomposites and CdS/dendrimer nanocomposites

synthesized via chemical or radiolytic routes.15,47 The PL spec-

trum consists of a peak around 310 nm originating from electron-

hole recombination (band edge emission) and a strong peak

around 497 nm due to recombination via surface localized state

(trap state emission). Band edge emission strongly depends on

particle size within the quantum confinement regime. Here, in the

present study, we have not observed any significant peak shift in

PL emission at 497 nm, when particle size was varied. Moreover,

Stokes’ shift is usually found to be small in case of band edge

emission.53–55 The absorption and emission spectra of the ZnTe/

dendrimer nanocomposites suggest that there is a substantial

Stokes’ shift from 329 to 497 nm. Thus, a size invariant PL

maxima and a large Stokes’ shift suggest that photoluminescence

originates from defect-states. However, the predominance of

trap emission suggests that the surface defects are not adequately

passivated. It is also possible that the majority of defects origi-

nate from the core atoms of the nanocrystals, which remain

inaccessible to the ligand-moieties of the dendrimer. The

maximum PLQE of 15% was obtained with NH2-terminated

dendrimer for the particle size of 2.5 nm, while the lowest PLQE

ce of amino (NH2), carboxyl (COOH), hydroxyl (OH), succinamic acid

Nanoscale, 2011, 3, 1139–1148 | 1143

Table 1 Average zeta potential and photoluminescence efficiency dataof ZnTe/dendrimer nanocomposites

Materials

Zetapotential(mV)

Photoluminescenceefficiency (%)

ZnTe_G4.NH2 +8.1 � 0.3 15.0ZnTe_G3.5COOH � 49.5 �

0.212.0

ZnTe_G4.OH �35 � 0.4 9.0ZnTe_G4.SAH �35.6 � 0.5 11.0

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(9%) was obtained for OH-terminated dendrimer (Table 1). It

can be concluded that NH2-, COOH- and SAH-terminated

dendrimers provide better protection than OH-terminated den-

drimer and yield higher luminescence quantum efficiencies.

Furthermore, the fluorescence image shows that, in contrast to

OH-terminated dendrimer, NH2-terminated dendrimer provides

better protection and produces highly luminescent ZnTe nano-

particles (Fig. 5a, b). Since dendrimers containing an almost

unlimited range of cores and peripheral groups may be synthe-

sized, it is now possible to easily control nanocluster properties

such as composition, size, morphology, solubility and degree of

encapsulation. One application of this latter property is to

control the release rate of entrained medicinal agents/sensors

based on structural or environmental changes, for targeted drug

delivery or in situ monitoring.

Fig. 5 (a) Fluorescence image of ZnTe/dendrimer nanocomposites using

NH2-terminated dendrimer (at 100� magnification). (b) Fluorescence

image of ZnTe/dendrimer nanocomposites using OH-terminated den-

drimer (at 100� magnification).

1144 | Nanoscale, 2011, 3, 1139–1148

3.4 Zeta potential

Zeta potential plays a crucial role, as the surface charge property

of a material influences the biological performance under phys-

iological conditions. By varying the surface charges one can vary

the electrostatic interaction, hydrophobic interaction and specific

chemical interaction between the NPs and various biological

entities for specific applications such as drug delivery, intracel-

lular targeting, cellular uptake, etc.15,56

The surface charge of the ZnTe/dendrimer nanocomposites

was measured by zeta potential, which depends on the terminal

groups of the dendrimer as shown in Table 1. Amine terminated

ZnTe/dendrimer nanocomposites were positively charged,

whereas carboxyl and succinamic acid terminated dendrimer

nanocomposites were negatively charged indicating that func-

tional groups of the dendrimer molecule is not significantly

influenced after the formation of the hybrid nanostructures. This

further indicates that after the formation of the hybrid nano-

structures the terminal amines of the dendrimers, which can be

used to link biological ligands, are still available. Importantly,

recent advances in using functionalized dendrimers for targeting,

imaging and drug molecules9,57 suggest that nanoparticles within

a dendrimer matrix are ideal nanodevices for a range of

biomedical applications.58,59 In addition, the surface charge

polarity of ZnTe/dendrimer nanocomposites is similar to the

corresponding dendrimer molecule. It can be concluded that the

dendrimer molecule imparts the surface charge tunability to the

ZnTe/dendrimer nanocomposites. It is expected that the higher

surface charge of the particles would help nanoparticles not to

aggregate and retain good colloidal stability through electro-

static repelling forces. However, nanocomposites using amine

terminated dendrimer possessing small positive charge are found

to be highly stable. However, it is difficult to make a comparison

between dendrimer terminal groups and the higher surface

potential of the particles in terms of the stabilization of the ZnTe

nanocrystal. For the matter of surface potential effect, earlier

work shows that fully acetylated Au nanoparticles in a dendrimer

matrix with a close to neutral surface potential still retain good

colloidal stability.6 Importantly, however, irrespective of the zeta

potential, the NPs remained stable in aqueous solutions, indi-

cating the strong colloidal stabilising effect of the dendrimer with

a large number of surface end groups.

3.5 Antimicrobial activity of semiconductor/dendrimer

nanocomposites

We tested the strains of V. cholerae, E. coli, S. aureus and B.

subtilis to evaluate the antibacterial activity of the ZnTe/den-

drimer nanocomposites and other II–VI semiconductor/den-

drimer nanocomposites. The strains of V. cholerae and E. coli

included in this study are multi-drug resistant. However, some of

the dendrimer nanocomposites tested here showed inhibitory

activity against V. cholerae O1 and ETEC by agar-diffusion

assay with significance (p < 0.05) (Table 2 and Fig. 6). However,

except one (CdTe_G4.NH2), none of the dendrimer nano-

composites was active against the Gram-positive bacteria used in

this study. CdTe_G4.NH2 was active against B. Subtilis but

exhibited bactericidal activity at very high concentrations of

more than 1000 mg ml�1. In contrast to semiconductor/dendrimer

This journal is ª The Royal Society of Chemistry 2011

Table 2 Antibacterial activity of nanocomposites determined by agar-diffusion method

Bacterial strains

Zone of inhibition diameter (mm)

Nanocomposites [50 ml (2 mg ml�1)/well]Antibiotic Control

ZnTe_G4.NH2 ZnTe_G3.5COOH ZnTe_ G3.5SAH Bulk ZnTe CdTe_ G4.NH2

Tetracycline[50 ml (0.2mg ml�1)/well] Water

V. cholerae NB2 14.6 � 0.3 11.3 � 0.4 11.8 � 0.4 19.3 � 0.3 13.3 � 0.4 20.0 � 0.0 0.0E. coli PC80 28.6 � 0.7 20.6 � 0.7 22.6 � 0.7 29.8 � 0.2 14.9 � 0.0 0.0 0.0B. subtillis ATCC 6623 0.0 0.0 0.0 0.0 12.1 � 0.0 21.0 � 0.2 0.0S. aureus ATCC 25923 0.0 0.0 0.0 0.0 0.0 ND 0.0

Fig. 6 Determination of the effect of nanocomposites on V. cholerae and

E. coli by agar-diffusion assay method. E. coli strain PC80 (A) and V.

cholerae strain NB2 (A) were spread on MHA. In each case, 50 ml of 2 mg

ml�1 of nanocomposites (in water) [wells 2, ZnTe_G4.NH2; 3,

ZnTe_G3.5COOH; 4, ZnTe_G3.5SAH; 6, Bulk ZnTe; 7, CdTe_G4.NH2;

and 8, Zn-Dendrimer complex], 50 ml of 0.2 mg ml�1 of tetracycline (wells

1, and 9), and 50 ml of water (wells 5 and 10) were added to the wells.

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nanocomposites, PAMAM dendrimer encapsulated silver

nanoparticles have good antimicrobial activity against Gram-

positive bacteria.60 CdS_G4.NH2 and ZnS_G4.NH2 nano-

composites were not active against any of the strains used in this

study.

This journal is ª The Royal Society of Chemistry 2011

The observed differences in antibacterial activity might be

attributed to the small particle size of ZnTe nanoparticles in the

dendrimer matrix. Large surface area and high penetrating

power make these particles effective to bind to the substrates on

the outer membrane and cell membranes of organisms. More-

over, ZnTe nanocrystals possess well-developed surface chem-

istry, chemical stability and appropriate smaller size (3–6 nm in

diameter, much smaller than a bacterium), which makes it easier

for them to penetrate the microorganism cell walls. Nanocrystals

are also able to maintain a constant shape and size in solution.

Despite the fact that the mechanism of the interaction between

nanoparticles and the constituents of the outer membrane/cell

wall of micro organisms are still unanswered, it might be that the

particles interact with the building elements of the outer

membrane/cell wall causing structural changes, degradation and

finally cell death. The semiconductor nanocomposites were most

effective against Gram-negative organisms, which may be

explained by the differences in the chemical nature of the mate-

rial present in outer membrane and cell wall.

For the determination of MIC and MBC for the dendrimer

nanocomposites against V. cholerae, and E. coli, the concentra-

tion ranges examined were from 2 to 1000 mg ml�1.

ZnTe_G4.NH2 had strong bactericidal activity with MIC

ranging from 64 to 128 mg ml�1 and MBC ranging from 128 to

256 mg ml�1 against V. cholerae and ETEC (Table 3). The

nanocomposites CdTe_G4.NH2, bulk ZnTe and ZnTe_G3.5-

COOH showed activity with MIC ranging from 256 to 512 mg

ml�1 and MBC 512 mg ml�1 against V. cholerae. CdTe_G4.NH2

also exhibited strong activity against ETEC. The results of the

time-kill studies are shown in Fig. 7. The MBC of ZnTe_G4.NH2

(128 mg ml�1) showed a 2.5-log reduction in growth of V. cholerae

in 8 h, compared to the untreated control. The time-kill kinetic

study demonstrated the time-dependent bactericidal activity of

ZnTe_G4.NH2. None of the active nanocomposites employed in

the present study released haemoglobin and hence, these were

not cytotoxic to human erythrocytes at concentrations of up to

8�MBC (see supplementary material). It was shown earlier that

cytotoxicity due to semiconductor quantum dots could be

correlated with surface oxidation probability and release of

heavy metals. In this context, it was also envisaged that appro-

priate coating could render quantum dot systems non-

toxic.39,61,62 It may reasonably be assumed from the observed

results that dendrimers offer better surface capping leading to the

non-cytotoxic nature of semiconductor/dendrimer nano-

composites.61,63 It appears from Table 2 that bulk ZnTe shows

a higher zone of inhibition as compared to nanocomposites.

Nanoscale, 2011, 3, 1139–1148 | 1145

Table 3 Minimum inhibitory concentration (MIC) and minimumbactericidal concentration (MBC) of nanocomposites against multi-drug-resistant strains of V. cholerae and E. coli

Nanocomposites

Antibacterial activity (mg ml�1)

V. cholerae NB2 E. coli PC80

MIC MBC MIC MBC

ZnTe_G4.NH2 64 128 128 256ZnTe_ G3.5COOH 256 512 512 1000Bulk ZnTe 512 512 ND NDCdTe_ G4.NH2 512 512 128 256Tetracycline 64 128 — —

Fig. 7 Cell growth of V. cholerae NB2 in the presence of ZnTe_G4.NH2

at MBC. –-–, and –C–, growth in the absence (control) and presence of

the nanoparticle (128 mg ml�1), respectively.

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However, if we consider the actual concentrations of ZnTe units

in nanocomposites in a given sample, it appears that this semi-

conductor unit contributes only 24% of the weight of the sample,

taking into account of large dendrimer molecule with molecular

weight of 14 200 (approx.). So, the comparison of effectiveness of

nanocomposites with bulk from Table 2 may not be appropriate.

This is evident from the higher MIC and MBC values of bulk

ZnTe in comparison with ZnTe/dendrimer nanocomposites.

Furthermore, the agar diffusion assay is one method for deter-

mining the antimicrobial susceptibility and interpretation of

results from this assay are analyzed based on the assumption that

antibiotics diffuse freely in the solid nutrient medium. However,

in many cases, this assumption may be incorrect, which leads to

significant deviations of the predicted behavior from the exper-

iment and to erroneous assessment of bacterial susceptibility to

antibiotics.64 Such significant deviation is observed during the

analysis of diffusion in solid agar of a number of antibiotics,

especially those of more hydrophobic or amphipathic nature.64

However, the broth micro dilution method is recognized as an

easy and reliable method for the determination of the MICs of

antibiotics and this assay has been suggested as a reference

method.65 In addition, synthesized semiconductor nano-

composites are highly water soluble and stable, whereas ZnTe is

poorly soluble in water. This limits the use of bulk ZnTe in any

biological system. Further, our studies also demonstrate that

bulk ZnTe is toxic to human erythrocytes whereas the synthe-

sized nanocomposites are non-toxic even at 8 times the minimum

bactericidal concentrations (MBCs). The rigid structure of the

dendrimer controls shape, size and stability of the nanocrystals

during interaction with the bacterial surface in solution phase.

During the present investigation, we observed that cysteine-

capped semiconductor nanocrystals such CdTe, CdS were not

effective in antimicrobial activity with regard to the strains in

question. This highlights the usefulness of semiconductor/den-

drimer nanocomposites for antimicrobial applications.

The ZnTe/dendrimer nanocomposite with amine terminal

group, ZnTe_G4.NH2 showed stronger antimicrobial activity

than the carboxyl terminated ZnTe/dendrimer nanocomposite,

ZnTe_G3.5COOH. The bacterial membranes contain mainly

anionic phospholipids and no cholesterol,66 while mammalian

cell membrane is mostly composed of the zwitterionic phos-

pholipids phosphatidylcholine and sphingomyelin, along with

cholesterol.67 Additionally, the outer membranes of Gram-

negative bacteria and cell wall of Gram-positive bacteria contain

1146 | Nanoscale, 2011, 3, 1139–1148

lipopolysaccharides and teichoic acid, respectively, which adds to

the negative charge of the bacterial surface.45,66 Amine termi-

nated ZnTe/dendrimer nanocomposites were positively charged;

whereas carboxyl terminated dendrimer nanocomposites were

negatively charged in the present study. It has been previously

demonstrated that the cationic nature of native antimicrobial

peptides clearly contributes to their preferential recognition by

the negatively charged outer surfaces of bacterial membranes.45,68

This type of electrostatic interaction between bacterial surfaces

and semiconductor nanoparticles may contribute to the better

effectiveness of ZnTe_G4.NH2 than ZnTe_G3.5COOH as both

have the identical dendrimer composites except their terminal

groups. In addition, increased antimicrobial activity of the

nanocomposites in the presence of amino terminated dendrimer

may be attributed to very high local concentration (128 amino

groups around a 46 �A diameter sphere) of nanoscopic size ZnTe

nanocomposite particles that are accessible for micro organisms.

However, interaction between the negatively charged dendrimer

nanocomposites and bacterial cell membrane remains a question

for further investigation.

4. Conclusion

In summary, we present here a simple approach for the synthesis

of monodisperse, water-soluble, multifunctional ZnTe/den-

drimer nanocomposites by a soft solution approach at low

temperature. The approach of using a multifunctional dendrimer

as a template to synthesize semiconductor nanoparticles could be

a useful general strategy for creating multifunctional materials

for a range of biomedical applications. The emergence of

multiple drug resistant pathogens is a serious clinical problem in

the treatment and containment of disease. However, this study

showed that the semiconductor nanoparticles had significant

bactericidal activity against multi-drug resistant enter-

opathogens including V. cholerae O1, the causative agent of the

dreadful disease cholera. The MBC of ZnTe_G4.NH2 (128 mg

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ml�1) showed a 2.5-log reduction in growth of V. cholerae in 8 h,

compared to the untreated control. Further, it may be mentioned

that the present study shows that sulfide based nanocomposites

are not effective as antimicrobial agents with regard to both

Gram-negative and Gram-positive bacteria studied. Thus, the

present investigation opens up the possibility of exploring suit-

able semiconductor/dendrimer nanocomposite for the develop-

ment of new class of antibacterial agents. There are some

questions that need to be addressed, such as, the exact mecha-

nism of interaction of nanocomposites with the bacterial cells

and the influence of surface area of nanoparticles in killing

activity.

Acknowledgements

One of the authors (S.G.) is thankful to the Council of Scientific

and Industrial Research, Govt. of India, for the award of Senior

Research Fellowship. The authors are also thankful to the Saha

Institute of Nuclear Physics, Kolkata for providing the electron

microscopy facility.

References

1 R. M. Crooks, B. I. Lemon, L. Sun, L. K. Yeung and M. Zhao, Top.Curr. Chem., 2000, 212, 81–135.

2 K. J. Landmark, S. Dimaggio, J. Ward, C. Kelly, S. Vogt, S. Hong,A. Kotlyar, A. Myc, T. P. Thomas, J. E. P. Hahn, J. R. Baker,M. M. Holl and B. G. Orr, ACS Nano, 2008, 4, 773–783.

3 S. Ghosh, S. C. Bhattacharya and A. Saha, Anal. Bioanal. Chem.,2010, 397, 1573–1582.

4 W. J. Scoot, O. M. Willson and R. M. Crooks, J. Phys. Chem. B, 2005,109, 692–704.

5 Y. Wang, X. Xie and T. Goodson 3rd, Nano Lett., 2005, 5, 2379–2384.6 L. Balogh, S. S. Nigavekar, B. M. Nair, W. Lesniak, C. Zhang,

L. Y. Sung, W. Kariapper, M. S. T. Tan, A. El-Jawahri, M. Llanes,B. Bolton, F. Mamou, A. Hutson, L. Minc and M. K. Khan,Nanomed.: Nanotechnol., Biol. Med., 2007, 3, 281–296.

7 A. P. Alivasatos, Science, 1996, 272, 933–937.8 J. M. Costa-Fern�andez, R. Pereiro and A. Sanz-Medel, TrAC, Trends

Anal. Chem., 2006, 25, 207–218.9 S. Svenson and D. A. Tomalia, Adv. Drug Delivery Rev., 2005, 57,

2160–2169.10 K. Sooklal, L. H. Hanus, H. J. Pleoehn and C. J. Murphy, Adv.

Mater., 1998, 10, 1083–1087.11 J. R. Lakowicz, I. Gryczynski, Z. Gryczynski and C. J. Murphy,

J. Phys. Chem. B, 1999, 103, 7613–7620.12 B. I. Lemon and R. M. Crooks, J. Am. Chem. Soc., 2000, 122, 12886–

12887.13 X. C. Wu, A. M. Bittner and K. J. Kern, J. Phys. Chem. B, 2005, 109,

230–239.14 S. K. Gayen, M. Brito, B. B. Das, G. Comanescu, X. C. Liang,

M. Alrubaiee, R. R. Alfano, C. Gonzalez, A. H. Byro,D. L. V. Bauer and V. Balogh-Nair, J. Opt. Soc. Am. B, 2007, 24,3064–3071.

15 S. Ghosh, A. Priyam and A. Saha, J. Nanosci. Nanotechnol., 2009, 9,6726–6735.

16 G. G. Luo, S. Lin, Y. J. Luo, D. Wu and S. N. Zheng, Chin. J. Inorg.Chem., 2005, 21, 1767–1771.

17 (a) S. Ghosh, A. Priyam, A. Chatterjee and A. Saha, J. Nanosci.Nanotechnol., 2008, 8, 952–957; (b) Y. Zenga, C. Tanga, G. Tiana,P. Yia, H. Huangb, N. Hub, S. Li, H. Huang, C. Li, B. Lin, X. Yu,Y. Ling and X. Xia, Chem. Eng. J., 2009, 156, 224–227.

18 (a) Y. D. Li, Y. Ding and Z. Y. Wang, Adv. Mater., 1999, 11, 847–850;(b) K. Yoshino, A. Memon, M. Yoneta, K. Ohmori, H. Saito andM. Ohishi, Phys. Status Solidi B, 2002, 229, 977–980.

19 (a) D. H. Lowndes, C. M. Rouleau, T. Thundat, G. Duscher,E. A. Kenik and S. J. Pennycook, Appl. Surf. Sci., 1998, 127–129,355–361; (b) L. Li, Y. Yang, X. Huang, G. Li and L. Zhang,J. Phys. Chem. B, 2005, 109, 12394–12398; (c) D. Lee, J. Zucker,

This journal is ª The Royal Society of Chemistry 2011

M. D. Divino, R. F. Austin, R. D. Feldman, K. L. Jones andA. M. Johnson, Appl. Phys. Lett., 1991, 59, 1867–1869.

20 Y. W. Jun, C. S. Choi and J. Cheon, Chem. Commun., 2001, 101–102.21 L. Li, Y. Yang, X. Huang, G. Li and L. Zhang, J. Phys. Chem. B,

2005, 109, 12394–12398.22 D. Lee, J. Zucker, M. D. Divino, R. F. Austin, R. D. Feldman,

K. L. Jones and A. M. Johnson, Appl. Phys. Lett., 1991, 59, 1867–1869.

23 U. Resch, H. Weller and A. Henglein, Langmuir, 1989, 5, 1015–1020.24 H. B. Huo, L. Dai, C. Liu, L. P. You, W. Q. Yang, R. M. Ma,

G. Z. Ran and G. G. Qin, Nanotechnology, 2006, 17, 5912–5915.25 D. H. Lowndes, C. M. Rouleau, T. Thundat, G. Duscher, E. A. Kenik

and S. Pennycook, Appl. Surf. Sci., 1998, 127–129, 355–361.26 (a) U. Resch, H. Weller and A. Henglein, Langmuir, 1989, 5, 1015–

1020; (b) H. B. Huo, L. Dai, C. Liu, L. P. You, W. Q. Yang,R. M. Ma, G. Z. Ran and G. G. Qin, Nanotechnology, 2006, 17,5912–5915.

27 Q. Li, S. Mahendra, D. Y. Lyon, L. Brunet, M. V. Liga, D. Li andP. J. J. Alvarez, Water Res., 2008, 42, 4591–4502.

28 Y. Zhang, H. Peng, W. Huanga, Y. Zhou and D. Yana, J. ColloidInterface Sci., 2008, 325, 371–376.

29 M. Rai, A. Yadav and A. Gade, Biotechnol. Adv., 2009, 27, 76–83.30 D. Wei, W. Sun, W. Qian, Y. Ye and X. Ma, Carbohydr. Res., 2009,

344, 2375–2382.31 S. Ray, R. Mohan, J. K. Singh, M. K. Samantaray, M. M. Shaikh,

D. Panda and P. Ghosh, J. Am. Chem. Soc., 2007, 129, 15042–15053.32 L. K. Adams, D. Y. Lyon and P. J. Alvarez, Water Res., 2006, 40,

3527–3532.33 K. Kasemets, A. Ivask, H. C. Dubourguier and A. Kahru, Toxicol. in

Vitro, 2009, 23, 1116–1122.34 V. Aruoja, H. C. Dubourguier, K. Kasemets and A. Kahru, Sci. Total

Environ., 2009, 400, 1461–1468.35 N. I. Chalmers, R. J. Palmer, L. Du-Thumm, R. Sullivan, W. Y. Shi

and P. E. Kolenbrander, Appl. Environ. Microbiol., 2007, 73, 630–636.

36 M. D. Hirschey, Y. J. Han, G. D. Stucky and A. J. Butler, JBIC, J.Biol. Inorg. Chem., 2006, 11, 663–669.

37 J. A. Kloepfer, R. E. Mielke and J. L. Nadeau, Appl. Environ.Microbiol., 2005, 71, 2548–2557.

38 Z. T. Jin, H. Q. Zhang, D. Sun, J. Y. Su and H. Sue, J. Food Sci., 2009,74, M46–M52.

39 Z. Lu, C. M. Li, H. Bao, Y. Qiao, Y. Toh and X. Yang, Langmuir,2008, 24, 5445–5452.

40 M. Kosek, C. Bern and R. L. Guerrant, Bull. World Health Organ.,2003, 81, 197–204.

41 G. B. Nair, T. Ramamurthy, S. K. Bhattacharya,A. K. Mukhopadhyay, S. Garg, M. K. Bhattacharya, T. Takeda,Y. Takeda and B. C. Deb, J. Infect. Dis., 1994, 169, 1029–1034.

42 L. Turvill, A. Wapnir, A. Wingertzahn, S. Teichberg and G. Farthing,Dig. Dis. Sci., 2000, 45, 946–951.

43 E. Salazar, J. Santisteban, E. Chea and M. Gutierrez, N. Engl.J. Med., 2000, 343, 463–467.

44 J. D. Williams, Int. J. Antimicrob. Agents, 2001, 18, 295–298.45 M. Stark, L. Liu and C. M. Deber, Antimicrob. Agents Chemother.,

2002, 46, 3585–3590.46 S. Pal, Y. K. Tak and J. M. Song, Appl. Environ. Microbiol., 2007, 73,

1712–1720.47 S. Ghosh, A. Datta and A. Saha, Colloids Surf., A, 2009, 355, 130–

138.48 P. Thakurta, P. Bhowmik, S. Mukherjee, T. K. Hajra, A. Patra and

P. K. Bag, J. Ethnopharmacol., 2007, 111, 607–612.49 D. S. Reeves, Antibiotic assays. In: P. M. Hawkey, D. A. Lewis (Eds)

Medical bacteriology: a practical approach. Oxford: IRL Press, 1989.pp. 195–221.

50 NCCLS, Approved standards M7–A4. Methods for dilutionantimicrobial susceptibility tests for bacteria that grow aerobically.National Committee for Clinical Laboratory Standards, Wayne, Pa,1997.

51 NCCLS, Methods for determining bactericidal activity of antimicrobialagents. Approved guideline M26-A. National Committee for ClinicalLaboratory Standards, Wayne, Pa, 1999.

52 H. Situ and L. A. Bobek, Antimicrob. Agents Chemother., 2000, 44,1485–1493.

53 S. V. Gaponenko, ‘‘Optical Properties of SemiconductorNanocrystals’’, Cambridge University Press, (1998).

Nanoscale, 2011, 3, 1139–1148 | 1147

Dow

nloa

ded

on 2

9 M

arch

201

1Pu

blis

hed

on 0

7 Ja

nuar

y 20

11 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C0N

R00

610F

View Online

54 Y. Tian, T. Newton, N. A. Kotov, D. M. Guldi and J. H. Fendler, J.Phys. Chem., 1996, 100, 8927–8939.

55 Z. Yu, J. Li, D. B. O’Connor, L. W. Wang and P. F. Barbara, J. Phys.Chem. B, 2003, 107, 5670–5674.

56 S. Patila, A. Sandbergb, E. Heckertc, W. Selfc and S. Seal,Biomaterials, 2007, 28, 4600–4607.

57 P. Singh, Dendrimer-based biological reagents: Preparation andapplications in diagnostics, in: J. M. J. Frechet, D. A. Tomalia(ed.), Dendrimer and Other Dendritic Polymers. New York: JohnWiley, 2001. pp. 463–84.

58 X. Shi, S. Wang, S. Meshinchi, M. V. Antwerp, X. Bi, I. Lee andJ. R. BakerJr, Small, 2007, 3, 1245–1252.

59 M. K. Khan, L. D. Minc, S. S. Nigavekar, M. S. T. Kariapper,B. M. Nair, M. Schipper, A. C. Cook, W. G. Lesniak andL. P. Balogh, Nanomed.: Nanotechnol., Biol. Med., 2008, 4, 57–69.

60 L. Balogh, D. R. Swanson, D. A. Tomalia, G. L. Hagnauer andA. T. McManus, Nano Lett., 2001, 1, 18–21.

1148 | Nanoscale, 2011, 3, 1139–1148

61 A. M. Derfus, W. C. W. Chan and S. N. Bhatia, Nano Lett., 2004, 4,111–118.

62 R. A. Hardman, Environ. Health Perspect., 2006, 114, 165–172.63 W. Lesniak, A. U. Bielinska, K. Sun, K. W. Janczak, X. Shi,

J. R. BakerJr and L. P. Balogh, Nano Lett., 2005, 5, 2123–2130.64 B. Bonev, J. Hooper and J. Parisot, J. Antimicrob. Chemother., 2008,

61, 1295–1301.65 (a) P. Luber, E. Bartelt, E. Genschow, J. Wagner and H. Hahn,

J. Clin. Microbiol., 2003, 41, 1062–1068; (b) L. Saiman, J. L. Burns,S. Whittier, J. Krzewinski, S. A. Marshall and R. N. Jones, J. Clin.Microbiol., 1999, 37, 2987–2991.

66 T. D. Brock, Biology of microorganisms. 2nd ed. Prentice-Hall, Inc.,Englewood Cliffs, N.J. 1974.

67 A. J. Verkleij, R. F. Zwaal, B. Roelofsen, P. Comfurius, D. Kastelijnand L. L. Van Deenen, Biochim. Biophys. Acta, Biomembr., 1973, 323,178–193.

68 Z. Oren and Y. Shai, Biopolymers, 1998, 47, 451–463.

This journal is ª The Royal Society of Chemistry 2011