ultrasensitive detection of toxic cations through...

ARTICLESPUBLISHED ONLINE: 9 SEPTEMBER 2012 | DOI: 10.1038/NMAT3406

Ultrasensitive detection of toxic cations throughchanges in the tunnelling current across films ofstriped nanoparticlesEun Seon Cho1,2†, Jiwon Kim3†, Baudilio Tejerina3, Thomas M. Hermans3, Hao Jiang4,Hideyuki Nakanishi3, Miao Yu2, Alexander Z. Patashinski3, Sharon C. Glotzer4, Francesco Stellacci1,2*and Bartosz A. Grzybowski3*

Although multiple methods have been developed to detect metal cations, only a few offer sensitivities below 1 pM, and manyrequire complicated procedures and sophisticated equipment. Here, we describe a class of simple solid-state sensors for theultrasensitive detection of heavy-metal cations (notably, an unprecedented attomolar limit for the detection of CH3Hg+ in bothstandardized solutions and environmental samples) through changes in the tunnelling current across films of nanoparticles(NPs) protected with striped monolayers of organic ligands. The sensors are also highly selective because of the ligand–shellorganization of the NPs. On binding of metal cations, the electronic structure of the molecular bridges between proximalNPs changes, the tunnelling current increases and highly conductive paths ultimately percolate the entire film. The nanoscaleheterogeneity of the structure of the film broadens the range of the cation-binding constants, which leads to wide sensitivityranges (remarkably, over 18 orders of magnitude in CH3Hg+ concentration).

Many cations are toxic, posing serious consequences forthe environment and for human health. As an example,organomercury species (for instance, methylmercury

(ii); refs 1,2) are extremely dangerous and cause neurologicaldisorders such as Minamata disease3,4, and cadmium poisoningresults in kidney and/or liver failure, bone softening and otherill effects (for example, Itai-itai disease5,6). Therefore, sensitiveand selective detection of toxic metal cations is of paramountimportance in assessing water pollution and in the eliminationof potentially serious health and environmental hazards7–9. Thereare multiple approaches to detect these ions, including atomicabsorption spectroscopy10,11 and inductively coupled plasmaspectroscopy12,13 (ICP), a host of optical methods (colorimetric14,15or fluorescence-based assays16,17 and systems based on surface plas-mon resonance18,19 or surface-enhanced Raman scattering20,21), andelectrochemical methods22,23. Frequently, however, these methodsrequire a sophisticated chemistry to incorporate host materials,such asmacrocyclic rings (crown ethers, calixarenes or porphyrins),metal-ion-recognizable proteins, nucleic acids and polymers.Moreover, the sensitivity of these methods is insufficient24–27in several situations of public health concern (for example,accumulation of low picomolar concentrations of CH3Hg+ andCd2+ in fish28,29). Furthermore, host–guest chemistries are notavailable for all cations, withCH3Hg+ being a prominent example2.

Here we show that gold nanoparticles (Au NPs) coatedwith binary mixtures of n-hexanethiol (HT) and alkanethiolsterminated with n = 1, 2 or 3 ethylene glycol (EG) units have astriped ligand structure (Fig. 1a,b) that generates supramolecular

1Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, 2Institute ofMaterials, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015, Switzerland, 3Department of Chemical and Biological Engineering and Department ofChemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA, 4Department of Chemical Engineering and Department ofMaterials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, USA. †These authors contributed equally to this work.*e-mail: [email protected]; [email protected].

pockets able to selectively trap ions, and that on such trappingthe electron-conduction mechanism through films of these NPschanges markedly30. We implement these findings in a solid-statesensor of unmatched sensitivity: HT/EG3 NPs sense CH3Hg+selectively and with a detection limit of ∼1 aM, HT/EG2 NPssense∼1 pM concentrations of Cd2+, and HT/EG1 NPs are∼1 nMsensors for Zn2+.

Au NPs (d = 4.7 ± 0.9 nm) were synthesized as describedpreviously31,32 (see Methods) and were functionalized with al-ternating stripes (Fig. 1a) of HT and EGn thiols (HS–(CH2)6–(OCH2CH2)n–OH, ultrapure grade, ProChimia). These stripedNPswere examined by scanning tunnelling microscopy (STM): thewidth of both HT and EG2 stripes was ∼1.3± 0.1 nm (Fig. 1b).For control experiments, we prepared NPs coated with disorderedHT/EGn mixed self-assembled monolayers (SAMs; random NPs(ref. 33)), but with the same relative content of the two thiols asin the striped NPs (see Supplementary Section S1), and also NPscovered with one-component SAMs of EGn thiols. All types of NPswere dispersed in methanol with a concentration of 2mM of Auatoms. To fabricate the sensors, Au electrodes (30 nm thick, 5mmlong) were sputter-coated onto a glass substrate using a shadowmask; the gap between the two electrodes was typically w = 50 µmwide and l = 5mm long (Fig. 1c). Approximately 3 µl of the AuNP solution was drop-cast onto the patterned glass and dried fortwo days in vacuum conditions to give NP films aboutH ∼ 150 nmthick (determined by profilometry for each sample); in this way,the number of NPs between the electrodes was of the order of1011 (Fig. 1d). To prevent re-dissolution of the film on exposure

NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 1© 2012 Macmillan Publishers Limited. All rights reserved.

http://www.nature.com/doifinder/10.1038/nmat3406

mailto:[email protected]

mailto:[email protected]

http://www.nature.com/naturematerials

ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT3406

=

Glass slide

Au electrode

A

A

1 mm

5 mm

5 mm

50 µm

Striped nanoparticle1,14-tetradecanedithiol

Glass slide

Au electrode

30 nm

j (A

m¬

2 )

j (A

m¬

2 )

E (kV m¬1) E (kV m¬1)

Before Zn2+

After Zn2+

Before CH3Hg+

After CH3Hg+

S

= O OHn

S

0

2

¬4

¬2

4

¬20 ¬10 0 10 20¬400

¬200

0

200

400

¬20 ¬10 0 10 20

a b

c

e

d( )

HT

EG

Figure 1 | Experimental set-up and typical j–E plots. a,b, An idealized scheme of a striped nanoparticle (a) and the corresponding STM image (here, forHT/EG2 NPs) obtained by a Veeco multimode scanning probe microscope on a vibration damping table (in an acoustic chamber, at room temperature, andin air; b). Scale bars, 10 nm (main image) and 5 nm (inset). c,d, The scheme and the dimensions of the device (c) and the side view of the NP film (d).e, Ohmic current density versus applied field, j–E, and dependencies for a film of striped HT/EG3 Au NPs before exposure to cations (black circles) andafter exposure to cations (coloured circles). In the graph on the left, the conductance of the films is virtually unchanged after immersion in a 1 mM solutionof Zn2+ (green circles). In the graph on the right, immersion of the same film in a 1 mM solution of CH3Hg+ (red circles) results in a marked change in theconductance (note the difference in the j ranges in the two plots).

to solvent, the film was reinforced by crosslinking the NPs with1,14-tetradecanedithiol (dithiol concentration: 5mM in toluene,crosslinking time: 40min; for details see refs 34–36). Subsequently,the films were washed with an excess of toluene, dried under astream of Ar, and then placed in vacuum conditions for two days.After drying the solvent, the Au electrodes were connected to ahigh-precision Keithley 6517 electrometer housed in a home-madeFaraday cage (to minimize the effects of static electricity). Thefilms were subsequently immersed in 2ml of solutions of K+, Na+,Zn2+, Cd2+, Cs+, CH3Hg+, Co2+, Hg2+, Pb2+, Cu2+, Ni2+, Tl+,Ca2+, Ag+ andCH3Zn+ chloride salts in ultrapure water (resistivity:18.2M� cm) with concentrations ranging from 1mM down to1 zM (zeptomolar, 10−21 M; note, the nature of the anions of thesalts used, for example, chlorides versus bromides, had no effecton the results described below). The films were kept immersed inthese solutions for t ∼ 120 s. This time was sufficient for the ionsto penetrate the full depth of the film, H—using a conservativeestimate of the diffusion coefficient of ions/small molecules ina nanoporous material, D ∼ 10−14 m2 s−1 (ref. 37), the diffusiondepth can be estimated as

√Dt ∼ 1 µm, which is greater than H .

Consequently, the conductivities of the films discussed below didnot change for longer immersion times. After immersion, the filmswere washed with copious amounts of water to remove excess saltand were then thoroughly dried under a stream of nitrogen. All

subsequent measurements were performed at room temperature(T ∼300±2K), under dry air in a hermetic Faraday cage, and in thepresence of desiccant (phosphorous pentoxide, P2O5; the presenceof the desiccant, however, had nonoticeable effect on the results).

The current density–applied field (j–E) dependencies for alltypes of films/cations were recorded for fields up to 20 kVm−1(higher fields cause irreversible changes in the NP films andcoalescence of the neighbouring nanoparticles30). In this range, thej–E dependencies were ohmic (Fig. 1e) and the conductivities of thefilms, σ = j/E , were constant for a given film type or condition (seeMethods and Supplementary Section S2 for typical j–V (t ) curvesat various concentrations of CH3Hg+). Before the immersion intothe salt solutions, the conductivities of the striped EGn particlefilms were σbefore∼2× 10−4�−1 m−1, which is close to the valuespreviously recorded for films of Au NPs covered with thiolateSAMs of comparable thickness30. The conductivities increased (toσafter) when the films were exposed to the cations (Fig. 1e). Thedegree of this increase depended crucially on the nature of theSAM coating the NPs, and on the nature and the concentration ofthe cations (Fig. 2). In all cases, the sensitivity of the sensors wasdefined as the concentration of cations in solution at which the ratioof the conductivities of the NP film, χ = σafter/σbefore, was greaterthan unity at the 99% confidence level (for statistical analysis, seeSupplementary Section S3).

2 NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials

© 2012 Macmillan Publishers Limited. All rights reserved.



NATURE MATERIALS DOI: 10.1038/NMAT3406 ARTICLES

Au NPs covered with HT/EG3 SAMs Au NPs covered with HT/EG2 SAMs Au NPs covered with HT/EG1 SAMs

NP types

CH3Hg+

CH3Hg+ controlCs+

Cs+

1

10

100

Cd2+

Cd2+

Cd2+

Cd2+

K+

K+

K+

K+

Na+

Na+

Na+

(Ran

dom

HT

/EG

1)

Na+

Zn2+

Zn2+

Zn2+

(R

ando

m H

T/E

G1)

Zn2+

Cs+

Cs+

Cs+

CH

3Hg+

CH

3Hg+

(0.1

mM

)

CH

3Hg+

CH

3Hg+

(0.1

mM

)

(0.1

mM

)

(0.1

mM

, ran

dom

HT

/EG

3)

(Ran

dom

HT

/EG

3)(R

ando

m H

T/E

G2)

(Ran

dom

HT

/EG

2)

1 mM

1 mM

1 mM

Ca2+

Ca2+

Ca2+

Hg2+

Hg2+

Hg2+

Cu2+

Cu2+

Cu2+

Pb2+

Pb2+

Pb2+

Ag+

Ag+

Ag+

Ni2+

Ni2+

Ni2+

Co2+

Co2+

Co2+

Tl+

Tl+

Tl+

CH

3Zn+

CH

3Zn+

CH

3Zn+

Cs+

1

10

10

1

10

1

=

afte

r/be

fore

Concentration (M)

100

1

10

σ σσσ

σσ

σσ

σσ

χ =

afte

r/be

fore

χ =

af

ter/

befo

reσ

σχ

=

afte

r/be

fore

χ

=

afte

r/be

fore

χ =

af

ter/

befo

reχ

10

1

Cd2+

Cd2+ controlK+

K+ control

Zn2+

Zn2+ controlNa+

Na+ control

a b

c d

e f

10¬24 10¬20 10¬16 10¬12 10¬8 10¬4

Concentration (M)10¬24 10¬20 10¬16 10¬12 10¬8 10¬4

Concentration (M)10¬24 10¬20 10¬16 10¬12 10¬8 10¬4

Figure 2 | Sensitivity (left column) and selectivity (right column) of cation sensing by different types of Au NPs decorated by HT/EGn SAMs.These measures are quantified by the change in the conductivities of the films on cation exposure, χ = σafter/σbefore. Filled symbols correspond tonanoparticles covered with striped HT/EGn SAMs; open symbols correspond to control experiments in which the nanoparticles were covered withdisordered/non-striped SAMs of the same composition. Standard deviations are for at least 12 independent experiments for each condition. The arrows inthe left column point to the detection limit/sensitivity (defined in the main text; see also Supplementary Section S3). a,b, Au NPs covered with HT/EG3

SAMs detect CH3Hg+ selectively with a detection limit of∼1 aM. c,d, HT/EG2 Au NPs detect Cd2+ and K+ with a detection limit of∼1 pM for bothcations. e,f, HT/EG1 Au NPs detect Zn2+ and also Na+ (detection limits:∼1 nM and∼1 µM, respectively). Note that the plots in a, c and e are doublelogarithmic and the conductivity increases nonlinearly with cation concentration.

Specifically, the HT/EG3 striped Au NPs detected methylmercury, CH3Hg+, with an approximately attomolar (10−18 M)sensitivity limit (corresponding to only ∼600 CH3Hg+ cations in1ml of the surrounding solution; Fig. 2a). As the concentration ofmethyl mercury increased, χ increased nonlinearly with [CH3Hg+]and reached a value of ∼75 at [CH3Hg+] = 0.1mM. Moreover,these sensing characteristics were selective for methyl mercury(Fig. 2b). For Co2+, Pb2+, Cu2+, Ni2+, Tl+, Ca2+, Ag+, CH3Zn+,Cd2+, K+, Na+, Cs+ and Zn2+, the conductivity did not change

perceptibly (χ ≈ 1) over the entire range of concentrationstested; for Hg2+ and Cs+ it changed only slightly (χ < 2.9and χ < 2.21, respectively). Furthermore, for Cs+, the detectionlimit was only ∼1 µM—that is, 12 orders of magnitude abovethat of CH3Hg+.

Films of Au NPs coated with striped SAMs of HT/EG2 detectedCd2+ with χ = 24.7 at 1mM and a detection limit ∼1 pM(Fig. 2c,d). These NPs, however, also detected K+ with χ = 10.2 at1mM and with ∼1 pM detection limit and, to a lesser degree, Ag+





Table 1 | Film sensitivity and energies in cation complexation.

NP types χbinding χnon-binding χmixture

HT/EG3 χCs+= 1.600±0.512 χNa+

=0.888±0.165 χCs+/Na+= 1.671±0.389

HT/EG2 χCd2+= 2.859±0.858 χZn2+

= 1.170±0.182 χCd2+/Zn2+= 2.373± 1.238

HT/EG1 χZn2+= 2.337±0.716 χCd2+

= 1.105±0.125 χZn2+/Cd2+= 2.406±0.383

Sensitivity (that is, χ =σafter/σbefore) in binary mixtures comprising cations that bind (denoted in bold) to the NPs and cations that do not bind (not bold). No significant differences between χbinding andχmixture were observed. Standard deviations are for at least six independent experiments for each condition.

with χ = 2.86. On the other hand, they did not bind Co2+, Pb2+,Cu2+, Ni2+, Tl+, Ca2+, CH3Zn+, Na+, Zn2+, Cs+ and CH3Hg+.Finally, Au NPs coated with striped SAMs of HT/EG1 were leastselective and detected Zn2+, Ag+ and Na+ (Fig. 2e,f). In this case,the values of χ were rather moderate (for example, at 1mM,6.51 for Zn2+, 5.00 for Ag+ and 2.72 for Na+) although thesensitivity towards Zn2+ remained relatively high (for example,detection limit ∼1 nM versus ∼1 µM for Na+). We note thatAu NPs coated with random/non-striped HT/EGn SAMs weresignificantly less sensitive and in most cases less selective indetecting the highest-χ cations (but not some lower-χ cations,see open symbols in Fig. 2). Also, particles coated only with pureEGn SAMs exhibited even lower values of χ (for details, seeSupplementary Section S4).

We emphasize that all χ values discussed above reflect the effectsof cation binding and not any water and/or drying effects. This wasverified in a series of experiments in which the films were washedin cation-free, deionized water and then dried—in this case, nostatistically significant changes in the conductivities of the filmswere observed after either one or multiple wash/dry cycles (forexample, for HT/EG2 AuNP films, χ =σafter/σbefore=1.011±0.029after one cycle and 1.013±0.031 after ten cycles). Also, when thecations were first captured into the films (such that conductivitiesincreased) but were then washed off (by placing the films in80 ◦C deionized water for 2–3 h), the conductivities returnedto their native values before cation capture (see SupplementarySection S7). We observe that the films owe their robustnessduring all of these experiments (see Supplementary Section S6)to the crosslinking of the NPs by dithiols; films in which theNPs were not crosslinked simply dissolved when washed withor immersed in water.

The changes in the conductivity of the film result from thecapture of the cations by the ligands on the NPs. This was directlyverified by means of the ICP and X-ray photoelectron spectroscopyanalyses of NPs before and after ion capture (see SupplementarySection S5 for these and other techniques used). In addition,Fourier transform infrared spectroscopy studies were performedthat evidenced spectral changes on cation binding. Of particularimportance here were the changes in the characteristic vibrationalmodes of the ethylene glycol units at 1001–1003 cm−1 and at827 cm−1. Remarkably, these changes were reversible in the sensethat the original spectra were retrieved when the cations werewashed off the NPs during immersion in 80 ◦C water (see above).These results also indicate that the capture of the cations ismediatedby coordination-type interactions with the EG units, as shouldindeed be expected on the basis of common chemical knowledge.It is also worth noting that cations for which χ = σafter/σbefore isclose to unity do not cause significant changes in the NP/NP–filmspectra as discussed in the Supplementary Section S5. Interestingly,a qualitatively similar result is obtained from solution experimentswhere the conductivities of NP suspensions are monitored duringtitration with different salt solutions. For those salts that arecharacterized by low χ in our solid-state (that is, NP–film)measurements, the conductance of NP solutions increases linearly

with the amount of salt added; for those, however, that exhibithigher χ in NP films, the solution conductance initially remainsconstant, indicating that the added cations are trapped on the NPs(and thus do not contribute to the conductance of the solution; seeSupplementary Section S5.5).

An essential feature of any chemical sensor is its selectivity, whichin our case needs to be determined on exposure ofNP films to cationmixtures.We first consider binarymixtures comprising cations thatthe NP films bind and sense with statistical significance (that is,giving conductance ratios χ = σafter/σbefore > 1 at 99% confidencelevels) and cations that do not bind strongly and do not changeconductivities at such confidence levels. Such pairs correspond tomost of the possible combinations among the cations we tested (seeFig. 2). The results for some representative pairs are summarized inTable 1. Specifically, for the HT/EG3 Au NP films, the presence ofweakly bindingNa+ does not significantly change the signal comingfrom stronger binding Cs+. Similarly, for HT/EG2 AuNP films, thepresence of Zn2+ does not strongly affect the signal due to Cd2+,and for HT/EG1 Au NP films the conductance on exposure tothe mixture of Zn2+ and Cd2+ is close to the value characterizingthe former cation. Such data confirm the selectivity of the filmstowards cations giving larger values ofχ .Moreover, the signals fromthe NP–film sensors are not affected by the changes in the ionicstrength of the solution due to the non-binding, low-χ cations (seeSupplementary Fig. S9a).

Of course, the selectivity decreases when the films are exposedto mixtures of cations characterized by similar χ > 1 values(Supplementary Section S12). This problem is the least pronouncedfor HT/EG3 Au NP films (see Fig. 2 and Supplementary SectionS8), which are therefore suitable for deployment as environmentalCH3Hg+ sensors. This is illustrated in Fig. 3, where we used oursensors to measure the CH3Hg+ content in environmental samplesdirectly relevant to the issues of public health—tap water, lakewater and fish (seeMethods for detailed preparation and analysis ofenvironmental samples). Importantly, the results we obtained forthe lake water could be directly compared to the values reportedin the literature for the same lake (Lake Michigan) sample, and theCH3Hg+ content in fish could be compared to the value determinedby the United States Geological Survey (USGS). For the lake water,our measurements gave a 5.9 fM concentration of CH3Hg+ (rangewithin one s.d. 0.35–43.3 fM) versus 5–210 fM reported in ref. 38.For the fish, we measured a concentration of 3.81 pM (rangewithin one s.d.: 1.62–8.19 pM), in excellent agreement with the3.58 pM value provided to us by the USGS after our experimentswere completed. A reassuring result—at least for the citizens ofEvanston—is that the content of CH3Hg+ in the tap water of thiscity is very low (27.7 aM, range within one s.d.: 4.5–138 aM). Wemake two further comments about all of these results. First, whenthe environmental samples were spiked with known concentrationsof CH3Hg+, the readings of our films fell onto the calibration curveobtained using the concentration standards (that is, solutions ofCH3Hg+ in pure, deionized water)—this means that there is nobackground interference from components other than CH3Hg+in the environmental samples. Second, we emphasize that our






Lake Michigan ( from ref. 38)Lake Michigan spikedTap waterFish ( from USGS)Fish blank spiked

} (reference samples)

Concentration CH3 Hg+ (M)

CH3 Hg+100

(¬

)

10

1

10¬23 10¬20 10¬17 10¬14 10¬11 10¬8 10¬5 10¬2

χ

(This work)

(This work)

Figure 3 | Selectivity of the films on exposure to cation mixtures andenvironmental samples. The χ values measured for the environmentalsamples (blue star: water from Lake Michigan, purple diamond: Evanstontap water, green triangle: mosquito fish collected by USGS from EvergladesNational Park). Red symbols correspond to the measurements of standardsolutions (that is, of known CH3Hg+ concentration in deionized water) andthe dashed calibration curve is the second order polynomial fit. Opensymbols correspond to the environmental samples ‘spiked’ with knownamounts of CH3Hg+. Pale green and pale blue regions correspond to theone-standard-deviation ranges in our measurements (three different films,at least six measurements per film) of, respectively, fish and lake watersamples. Dark green horizontal bar corresponds to the range of CH3Hg+

content in the mosquito fish determined by USGS, and the dark bluehorizontal bar corresponds to the one-standard-deviation range ofCH3Hg+ concentration in Lake Michigan determined in ref. 38. For furtherexperimental detail, see Methods.

results were achieved in a technically straightforward manner(by simply dipping the film in the sample solution), in contrastto other methods that require tedious sample pretreatment,including conversion of methylmercury to ethylmethylmercuryand concentration on a dedicated column before analysis by gaschromatography combinedwith ICPmass spectrometry2,39.

We now turn our attention to the fundamental aspects ofthe relationship between the binding of cations into the filmand the observed current changes. There are two interrelatedcomponents that need to be considered: cation-facilitated electrontransport through the molecular bridge between proximal NPs,and formation of the conductive paths through the film, whichultimately give rise to the high sensitivity of the cation detection.To understand the electronic structure of the bridge, we performedquantum-mechanical calculations for a fragment of NP surfacecomprising two nearby EGn stripes and an HT stripe betweenthem (Fig. 4a). Calculations were performed for both the cation-free HT/EGn systems, as well as a system in which differentmetal cations were placed either within the EGn stripes (denotedintra in Table 2) or at the edges of these stripes (inter). In allcases, the geometry of the system was optimized by a semi-empirical quantum-mechanical method NDDO-PM6 (ref. 40; withtheMOPAC2007 (ref. 41) software) at the Non-equilibrium EnergyResearch Center supercomputer centre. The calculations indicatethat the experimentally observed selectivities do not correlate withthe calculated energies of the complexation of the cations by variousHT/EGn AuNPs or with the heats of formation of these complexes,nor with the energies of the highest occupied molecular orbital(HOMO) or the lowest unoccupied molecular orbital (LUMO).On the other hand, the observed cation selectivities can be relatedto the HOMO/LUMO gap, 1E (Table 2). This result suggeststhat both HOMO and LUMO orbitals are involved in electrontransport. Although this is not a typical scenario in describing

Table 2 | Film sensitivity and energies in cation complexation.

NP/cation types HT/EG1 HT/EG2 HT/EG3

Free 9.160 9.143 9.142Zn2+ Intra 1.834 2.408 3.098

Inter 1.398 1.976 3.031Cd2+ Intra 4.395 3.969 4.088

Inter 3.928 3.676 4.243CH3Hg+ Intra 6.045 6.392 6.272

Inter 6.154 6.129 6.045

Calculated energies of the HOMO/LUMO gaps (in eV) for different cations and EGn thiols.‘Interband’ corresponds to the situation where the cations are bound at the edge of an EGnstripe; ‘intraband’ corresponds to the cation being placed within one EGn band. Entries in boldcorrespond to minimal gap energies for a given type of EGn and generally agree with/reflect thecation-binding selectivity observed in experiments.

conduction through a molecular bridge42, similar models havebeen used previously to describe transport through disorderedmedia (as our films are)43. In this context, we observe that inour system the HOMO orbital (and other high-energy occupiedorbitals) is always located close to the NP surface (Fig. 4b), whereasthe LUMO orbital (and unoccupied orbitals close to it) is mostlycentred on the EGn/cation fragments (Fig. 4c). Therefore, for theelectrons to travel through the SAM, they must be injected fromthe HOMO into the LUMO. Importantly, the energy required forthis injection is substantially diminished on cation binding, andthe smallest values of 1E generally reflect the detection specificityobserved in experiments: for Zn2+, the smallest 1E correspondsto the binding to the HT/EG1 Au NPs; Cd2+ has the smallest1E when binding to HT/EG2 Au NPs; for CH3 Hg +, 1E is theleast when binding inter-stripe to HT/EG3 Au NPs (although, inthis case, calculations give the same value for HT/EG1 particles;Table 2). We note that for any given HT/EGn/cation system, the1E gaps (and the absolute binding energies) are typically lowerwhen cations coordinate with ethylene glycols at the edges of theEGn stripes than when these cations are fully buried within the EGnstripes. This observation suggests that the free space provided bythe HT stripes is important for the operation of our sensors (asalso corroborated by molecular dynamics simulations illustrated inFig. 4d and Supplementary Section S9).

The decrease in 1E on cation binding increases the electronicconductance of a NP-SAM//SAM-NP molecular bridge, σc ∝(e2/h)(e−2(m1E)1/2L/h)2, where h is Planck’s constant, m is the restmass of an electron and L is the thickness of the NP ligandshell44. Importantly, as multiple cations bind into the NP film,the local changes over individual bridges can translate into aglobal change in the conductance of the film. This processcan be described by combining σc with the cation-bindingkinetics and Bruggeman’s effective medium theory45,46 (EMT)accounting for the formation of conductive paths percolatingthe NP film (Fig. 5a,b). The binding of cations to the EGnligands as a function of concentration [C]0 can be describedin terms of the Sips isotherm47: θ = (Keq[C]0)a/[1+ (Keq[C]0)a],where θ is the fraction of occupied sites, Keq is the equilibriumassociation constant and a is the heterogeneity index. The Sipsisotherm is a composite of the familiar Langmuir isotherm(θLM = KLM[C]0/[1 + KLM[C]0]) and the Freundlich isotherm(θF=KF[C]

1/n0 ), commonly used for low-concentration binding in

heterogeneous systems. The Sips isotherm approaches Langmuir’sformula for a close to unity and Freundlich’s equation at lowconcentration, and has been used to describe ligand binding topowders48 and aggregated nanoparticle media49. The heterogeneityindex (a) determines the width of the Gaussian-like distributionof Keq, which arises from steric effects, conformational changesand differences in the chemical microenvironment50. Using the





HTEGn EGn

a b

c

d

Figure 4 | Rationalizing the conductance through the molecular bridge between the striped NPs. a, Top view of the modelled system comprising 15 HTand 4 EGn thiols arranged in alternating stripes on the Au(111) surface with a (

√3×√

3) R30◦ lattice distribution of the HT/ EGn ligands. b,c, HOMO (b)and LUMO (c) orbitals calculated for the HT/EG3 Au NPs capturing CH3Hg+. Note that the HOMO is closer to the surface of the nanoparticle (localizedon the lone pairs of the sulphur atoms), whereas the LUMO is centred on the EGn/cation complexes (corresponding to the anti-bonding σ ∗ (O–M) of theEGn/cation complexes). These simulations are performed for nanoparticles in vacuum, which is appropriate for the experimental measurements beingperformed in dry films. d, A snapshot from the molecular dynamics simulations showing a typical ion-trapping scenario where the cation (green ball) istrapped near the edge of an EGn stripe (for further details, see Supplementary Section S9).

expressions for σc and θ , the EMT formula describing the changein the conductance of the film on cation binding becomes (seeSupplementary Section S11):

χS-EMT=1

(z−2)

{(12

z(Keq[C]0

)a1+

(Keq[C]0

)a −1)σc,n

+

(12z

(1−

(Keq[C]0

)a1+

(Keq[C]0

)a)−1

)

+

[((12

z(Keq[C]0

)a1+

(Keq[C]0

)a −1)σc,n

+

(12z

(1−

(Keq[C]0

)a1+

(Keq[C]0

)a)−1

))2

+2(z−2)σc,n

1/2 (1)

where σc is normalized by dividing by the conductance of thefilm before binding cations (σc,n= σc/σbefore), and z is the numberof high-conductance bonds any NP can make to its neighbours.Figure 5c illustrates that with the experimentally measured (byisothermal titration calorimetry, see Supplementary Section S10)values of Keq, this model reproduces the essential features of thenonlinear cation concentration versus conductivity dependenciesrecorded in experiments. As expected, the fitted values of a and z for

both Cd2+ and K+ are nearly equal (for Cd2+: a= 0.42±0.32 andz = 2.1± 0.5; for K+: a= 0.42± 0.55 and z = 1.9± 1.0), becausethey reflect the intrinsic nanostructure of the Au NP films. Thevalues of z ∼ 2 suggest that there are relatively few possibilitiesto form high-conductance connections between neighbouring NPsin the film (z = 6 would signify that such connections can bemade between all nearest-neighbour NPs in a randomly packedfilm)46. More importantly, the values of a< 1 confirm that thereis significant heterogeneity of the cation binding to the NP film(unlike in isothermal titration calorimetry solution studies forwhich a = 1). This result agrees with previous works on thenanoenvironment47, and it endows our sensor with a remarkablywide range of sensitivity. If the cation binding properties in thefilms were the same as those observed in solution (a = 1), thesensitivity curves would look like the dashed curve in Fig. 5c.As seen, this curve transitions sharply between constant χminand χmax values over a relatively narrow range of concentrations(∼4 orders of magnitude). In contrast, with the heterogeneityeffects being operative, the ranges over which χ values changeare much wider, ∼9 orders of magnitude for Cd2+ and K+ andeven more (∼18 orders) for CH3Hg+. Although the broadeningof binding energies is generally considered a disadvantage thatcomplicates affinity studies50, it is in fact the key—and unique—principle underlying the remarkable range of sensing of our NPdevices. In other words, the packing of chemically identical NPsinto a nanostructured thin film gives rise to adsorption siteswith not a single but many different energies (and thus Keq






100

Langmuir

Cd2+

K2+ ( fit S-EMT)

Concentration (M)

(¬)

10

1

χ

( fit S-EMT)

10–17 10–13 10–9 10–5 10–1

a c

b

Figure 5 | Percolation of Au NP films on cation binding. a, Schematic representation of a film of striped Au NPs, between two Au electrodes. b, On bindingof cations (blue spheres) conductive paths (blue line) connecting the two electrodes percolate the film and connect the two electrodes. c, Sensitivity (thatis, χ = σafter/σbefore) of HT/EG2 Au NP films on binding of K+ and Cd2+ from solutions of different concentrations. Symbols correspond to theexperimental data (the error bars are based on 12 independent measurements for each condition). The solid lines are theoretical fits based on the modeldescribed in the main text (equation (1)). The values of Keq parameters were determined calorimetrically (see Supplementary Section S10) and wereKCd2+

eq = 2.75±0.31× 106 M−1 for Cd2+ and KK+eq = 1.5±0.12× 106 M−1 for K+. As a reference, the Langmuir isotherm (a= 1) for Cd2+ is also included

(dashed black line).

values). The sites with high Keq values are the first ones to beoccupied at very low concentration, whereas the low Keq sitessaturate at much higher concentrations. This effect grants theNP films with both a low onset and, at the sametime, a verybroad range of detection.

Overall, the high sensitivity of our solid-state sensors derivesfrom a subtle interplay between cation/striped-NPmacromolecularrecognition, electron transport over the NP/cation/NP bridges,and the formation of conductive paths through heterogeneous NPfilms. Together, these phenomena provide a new principle forsolid-state sensing and enable inexpensive, sensitive and portableenvironmental sensors. With further tailoring—both in terms ofligand chemistries and stripe dimensions—it should be possible toextend the applicability of our nanostructured materials to othersensing systems whose properties derive from macromolecularrecognition on NP surfaces.

MethodsSynthesis of striped Au NPs. Striped Au NPs covered by 1-hexanethiol(Sigma-Aldrich 739286) and 1-hexanethiol terminated in n= 1–3 units of ethyleneglycol (EGn, ProChimia) (n= 1, ProChimia TH 001-m6.n1-1; n= 2, ProChimiaTH 001-m6.n2-1; n=3, ProChimia TH 001-m6.n3-1, respectively) in 1:3 ratio weresynthesized by a modified Stucky method51. Briefly, 0.45mmol of AuPPh3Cl wasmixed with 0.45mmol of thiol mixture in 40ml of chloroform/dimethylformamideco-solvent, and 5mmol of tert-butylamine-borane complex was added. Thereactant was heated to 55 ◦C and stirred for 1 h. After cooling to room temperature,200ml of chloroform was added to precipitate Au NPs, and the product wascentrifuged and washed with chloroform. The ligand shell structure was determinedusing STM as described previously (see Fig. 1b in the main text)32,51–54. Furthercharacterization is included in Supplementary Section S5.

Film conductivities. The film conductivities of the HT/EGn Au NP filmsbefore capturing cations, σbefore, were 1.9× 10−4± 1.6× 10−4 �−1 m−1,1.4× 10−4 ± 8.7× 10−5 �−1 m−1 and 2.7× 10−4 ± 1.3× 10−4 �−1 m−1for n= 1, n= 2 and n= 3, respectively. These values were calculated asσbefore = I/wt ×1/V × l = j/V × l , where I and j are current (A) and currentdensity (Am−2), respectively, V is applied voltage (V), and w , t and l are thewidth, thickness and gap of the Au electrode, respectively (w = 5mm, t = 30 nmand l= 50 µm). The conductivity values and their s.d. are based on at least 12different material samples for each type of a film. We note that these conductivityvalues are comparable to those measured for films of Au NPs covered with thiolsof similar thickness30,55.

Preparation and analysis of environmental samples. To prevent any externalcontamination of the samples, all containers (glass vials, VWR) were cleaned

in 4M aqueous HCl solution at 65 ◦C (for 12 h) and rinsed three times withcopious amounts of deionized water (resistivity: 18.2M� cm). All experimentswere performed in a LabConco biosafety class II laminar flow cabinet with ahigh-efficiency particulate arresting (HEPA) 99.99% filter in a HEPA-filteredroom, similar to class 100 cleanroom conditions. Water from Lake Michiganwas sampled at Northwestern University (Evanston Campus) private beachinto a clean polypropylene tube (VWR). Tap water was sampled from a waterfountain in the Technological Institute building at Northwestern University.Fish samples were obtained from the USGS, Mercury Research Laboratory(http://wi.water.usgs.gov/mercury-lab/index.html). To prepare fish samples foranalysis, around 10mg of lyophilized mosquito fish from Everglades National Parkcollected by the USGS was exactly weighed and digested using 10ml of 5M HNO3

for 8 h at 65 ◦C and then neutralized using 5M aqueous KOH solution (note: thepH was checked by a pH meter to confirm that the solution was neutralized—ourAu NP film is unstable in very acidic solutions). The solution was then diluted1,000 times to bring the background salt (KNO3) concentration down to themillimolar range. For spiking the blank fish and lake water samples, 10% (v/v) of10−12 M,10−8 Mand 10−4 Mof CH3Hg+ solution were added.

Received 15 September 2011; accepted 20 July 2012;published online 9 September 2012

References1. Boening, D. W. Ecological effects, transport, and fate of mercury: A general

review. Chemosphere 40, 1335–1351 (2000).2. Clarkson, T. W. The toxicology of mercury. Crit. Rev. Clin. Lab. Sci. 34,

369–403 (1997).3. Matsumoto, H., Koya, G. & Takeuchi, T. Fetal minamata disease: A

neuropathological study of two cases of intrauterine intoxication by a methylmercury compound. J. Neuropathol. Exp. Neurol. 24, 563–574 (1965).

4. Harada, M. Minamata disease—methylmercury poisoning in Japan caused byenvironmental pollution. Crit. Rev. Toxicol. 25, 1–24 (1995).

5. Inaba, T. et al. Estimation of cumulative cadmium intake causing Itai-itaidisease. Toxicol. Lett. 159, 192–201 (2005).

6. Nomiyama, K. Recent progress and perspectives in cadmium health-effectsstudies. Sci. Total Environ. 14, 199–232 (1980).

7. Nriagu, J. O. & Pacyna, J. M. Quantitative assessment of worldwidecontamination of air, water and soils by trace-metals. Nature 333,134–139 (1988).

8. Lin, S. H. & Juang, R. S. Heavy metal removal from water by sorption usingsurfactant-modified montmorillonite. J. Hazard. Mater. 92, 315–326 (2002).

9. Jensen, S. & Jernelov, A. Biological methylation of mercury in aquaticorganisms. Nature 223, 753–754 (1969).

10. Narin, I., Soylak, M., Elci, L. & Dogan, M. Determination of trace metal ionsby AAS in natural water samples after preconcentration of pyrocatechol violetcomplexes on an activated carbon column. Talanta 52, 1041–1046 (2000).

11. Ghaedi, M., Ahmadi, F. & Shokrollahi, A. Simultaneous preconcentration anddetermination of copper, nickel, cobalt and lead ions content by flame atomicabsorption spectrometry. J. Hazard. Mater. 142, 272–278 (2007).



http://wi.water.usgs.gov/mercury-lab/index.html



12. Caroli, S., Forte, G., Iamiceli, A. L. &Galoppi, B. Determination of essential andpotentially toxic trace elements in honey by inductively coupled plasma-basedtechniques. Talanta 50, 327–336 (1999).

13. Demuth, N. & Heumann, K. G. Validation of methylmercury determinationsin aquatic systems by alkyl derivatization methods for GC analysis usingICP-IDMS. Anal. Chem. 73, 4020–4027 (2001).

14. Lee, J. S., Han, M. S. & Mirkin, C. A. Colorimetric detection of mercuricion (Hg2+) in aqueous media using DNA-functionalized gold nanoparticles.Angew. Chem. Int. Ed. 46, 4093–4096 (2007).

15. Huang, C. C. & Chang, H. T. Parameters for selective colorimetric sensing ofmercury(II) in aqueous solutions using mercaptopropionic acid-modified goldnanoparticles. Chem. Commun. 1215–1217 (2007).

16. Chiang, C. K., Huang, C. C., Liu, C. W. & Chang, H. T. Oligonucleotide-basedfluorescence probe for sensitive and selective detection of mercury (II) inaqueous solution. Anal. Chem. 80, 3716–3721 (2008).

17. Chang, H. Y., Hsiung, T. M., Huang, Y. F. & Huang, C. C. Using rhodamine6G-modified gold nanoparticles to detect organic mercury species in highlysaline solutions. Environ. Sci. Technol. 45, 1534–1539 (2011).

18. Kang, T., Hong, S. R., Moon, J., Oh, S. & Yi, J. Fabrication of reusablesensor for detection of Cu2+ in an aqueous solution using a self-assembledmonolayer with surface plasmon resonance spectroscopy. Chem. Commun.3721–3723 (2005).

19. Li, D., Wieckowska, A. & Willner, I. Optical analysis of Hg2+ ions byoligonucleotide-gold-nanoparticle hybrids and DNA-based machines. Angew.Chem. Int. Ed. 47, 3927–3931 (2008).

20. Wang, G. Q., Wang, Y. Q., Chen, L. X. & Choo, J. Nanomaterial-assistedaptamers for optical sensing. Biosens. Bioelectron. 25, 1859–1868 (2010).

21. Yin, J. et al. SERS-active nanoparticles for sensitive and selective detection ofcadmium ion (Cd2+). Chem. Mater. 23, 4756–4764 (2011).

22. Lin, Z. Z., Li, X. H. & Kraatz, H. B. Impedimetric immobilized DNA-basedsensor for simultaneous detection of Pb2+, Ag+, and Hg2+. Anal. Chem. 83,6896–6901 (2011).

23. Zhang, Z., Yu, K., Bai, D. & Zhu, Z. Q. Synthesis and electrochemical sensingtoward heavymetals of bunch-like bismuth nanostructures.Nanoscale Res. Lett.5, 398–402 (2010).

24. Thompson, R. B., Maliwal, B. P., Feliccia, V. L., Fierke, C. A. & McCall, K.Determination of picomolar concentrations of metal lens using fluorescenceanisotropy: Biosensing with a ‘reagentless’ enzyme transducer. Anal. Chem. 70,4717–4723 (1998).

25. Forzani, E. S., Zhang, H. Q., Chen, W. & Tao, N. J. Detection of heavy metalions in drinking water using a high-resolution differential surface plasmonresonance sensor. Environ. Sci. Technol. 39, 1257–1262 (2005).

26. Kumar, M. & Zhang, P. Highly sensitive and selective label-free opticaldetection of mercuric ions using photon upconverting nanoparticles.Biosens. Bioelectron. 25, 2431–2435 (2010).

27. Song, H. D. et al. Picomolar selective detection of mercuric ion (Hg2+) usinga functionalized single plasmonic gold nanoparticle. Nanotechnology 21,145501 (2010).

28. Hinkle, P. M., Kinsella, P. A. &Osterhoudt, K. C. Cadmiumuptake and toxicityvia voltage-sensitive calcium channels. J. Biol. Chem. 262, 16333–16337 (1987).

29. Rashed, M. N. Cadmium and lead levels in fish (Tilapia nilotica) tissuesas biological indicator for lake water pollution. Environ. Monit. Assess. 68,75–89 (2001).

30. Nakanishi, H. et al. Photoconductance and inverse photoconductance in filmsof functionalized metal nanoparticles. Nature 460, 371–375 (2009).

31. DeVries, G. A. et al. Divalent metal nanoparticles. Science 315, 358–361 (2007).32. Jackson, A. M., Myerson, J. W. & Stellacci, F. Spontaneous assembly of

subnanometre-ordered domains in the ligand shell of monolayer-protectednanoparticles. Nature Mater. 3, 330–336 (2004).

33. Witt, D., Klajn, R., Barski, P. & Grzybowski, B. A. Applications properties andsynthesis of omega-functionalized n-alkanethiols and disulfides—the buildingblocks of self-assembled monolayers. Curr. Org. Chem. 8, 1763–1797 (2004).

34. Kowalczyk, B., Apodaca, M. M., Nakanishi, H., Smoukov, S. K. & Grzybowski,B. A. Lift-Off and micropatterning of mono- and multilayer nanoparticle films.Small 5, 1970–1973 (2009).

35. Klajn, R., Bishop, K. J. M. & Grzybowski, B. A. Light-controlled self-assemblyof reversible and irreversible nanoparticle suprastructures. Proc. Natl Acad.Sci. USA 104, 10305–10309 (2007).

36. Kowalczyk, B., Lagzi, I. & Grzybowski, B. A. Nanoarmoured droplets ofdifferent shapes formed by interfacial self-assembly and crosslinking of metalnanoparticles. Nanoscale 2, 2366–2369 (2010).

37. Han, S. B. et al. Chromatography in a single metal-organic framework (MOF)crystal. J. Am. Chem. Soc. 132, 16358–16361 (2010).

38. Sullivan, K. A. & Mason, R. P. The concentration and distribution of mercuryin Lake Michigan. Sci. Total Environ. 213, 213–228 (1998).

39. Jackson, B., Taylor, V., Baker, R. A. & Miller, E. Low-level mercury speciationin fresh waters by isotope dilution GC-ICP-MS. Environ. Sci. Technol. 43,2463–2469 (2009).

40. Pople, J. & Beveridge, D. Approximate Molecular Orbital Theory(McGraw-Hill, 1970).

41. Stewart, J. J. P. Stewart Computational Chemistry (Colorado Springs, 2007).42. Nitzan, A. Chemical Dynamics in Condensed Phases : Relaxation, Transfer and

Reactions in Condensed Molecular Systems Ch. 16 (Oxford, 2006).43. Burlakov, V. M. et al. Discrete hopping model of exciton transport in

disordered media. Phys. Rev. B 72, 075206 (2005).44. Holmlin, R. E. et al. Electron transport through thin organic films in

metal-insulator-metal junctions based on self-assembled monolayers. J. Am.Chem. Soc. 123, 5075–5085 (2001).

45. Landauer, R. The electrical resistance of binary metallic mixtures. J. Appl. Phys.23, 779–784 (1952).

46. Kirkpatr, S. Classical transport in disordered media—scaling andeffective-medium theories. Phys. Rev. Lett. 27, 1722–1725 (1971).

47. Sips, R. On the structure of a catalyst surface. J. Chem. Phys. 16, 490–495 (1948).48. Kumar, K. V. & Porkodi, K. Relation between some two- and three-parameter

isotherm models for the sorption of methylene blue onto lemon peel.J. Hazard. Mater. 138, 633–635 (2006).

49. Hristovski, K., Baumgardner, A. & Westerhoff, P. Selecting metal oxidenanomaterials for arsenic removal in fixed bed columns: Fromnanopowders to aggregated nanoparticle media. J. Hazard. Mater. 147,265–274 (2007).

50. Vijayendran, R. A.&Leckband,D. E. A quantitative assessment of heterogeneityfor surface-immobilized proteins. Anal. Chem. 73, 471–480 (2001).

51. Verma, A. et al. Surface-structure-regulated cell-membrane penetration bymonolayer-protected nanoparticles. Nature Mater. 7, 588–595 (2008).

52. Jackson, A. M., Hu, Y., Silva, P. J. & Stellacci, F. From homoligand- tomixed-ligand-monolayer-protected metal nanoparticles: A scanning tunnelingmicroscopy investigation. J. Am. Chem. Soc. 128, 11135–11149 (2006).

53. Kuna, J. J. et al. The effect of nanometre-scale structure on interfacial energy.Nature Mater. 8, 837–842 (2009).

54. Centrone, A. et al. The role of nanostructure in the wetting behavior ofmixed-monolayer-protected metal nanoparticles. Proc. Natl Acad. Sci. USA105, 9886–9891 (2008).

55. Zabet-Khosousi, A. & Dhirani, A. A. Charge transport in nanoparticleassemblies. Chem. Rev. 108, 4072–4124 (2008).

AcknowledgementsThis work was supported by the Non-equilibrium Energy Research Center, which isan Energy Frontier Research Center funded by the US Department of Energy, Officeof Science, Office of Basic Energy Sciences under grant number DE-SC0000989. E.S.C.and F.S. acknowledge the support of ENI within the MIT Energy initiative for theirwork. E.S.C. is supported by a Samsung Scholarship from the Samsung Foundation ofCulture. H.J., S.C.G. and F.S. acknowledge support from the Defense Threat ReductionAgency under Grant No. HDTRA1-09-1-0012. T.M.H. is financially supported by theHuman Frontier Science Program.We acknowledge L. Meda for her X-ray photoelectronspectroscopy measurements, and R. Borrelli and P. Cesti for helpful discussions. We alsothank the USGS for providing fish samples and for helpful discussions.

Author contributionsE.S.C. synthesized the striped nanoparticles, performed all of their characterizationbefore and after binding to ions, and helped with some of the solid-state work.; J.K.made and characterized all sensors, performed cation capture experiments, measuredthe conductivities of all sensors and analysed data; B.T. developed quantum-mechanicalmodels and ran quantum-mechanical calculations; T.M.H. developed and validatedthe binding/percolation models; H.J. ran and analysed MD simulations; S.C.G.analysed and supervised the MD simulations; H.N. performed initial experiments withNP films; M.Y. performed the STM characterization of the particles; A.Z.P. helpedwith the theory of percolation phenomena; F.S. and B.A.G. conceived the ideas andsupervised the research.

Additional informationSupplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints. Correspondenceand requests for materials should be addressed to F.S. or B.A.G.

Competing financial interestsThe authors declare no competing financial interests.





http://www.nature.com/reprints


ultrasensitive detection of toxic cations through...

Documents