Sensing of the uranyl ion based on its complexationwith bisphosphonate-capped gold nanoparticles

Nicolas Drogat a, Loïc Jauberty a,b, Vincent Chaleix a, Robert Granet a, Erwann Guénin c,Vincent Sol a,n, Vincent Gloaguen a

a Laboratoire de Chimie des Substances Naturelles, EA 1069, Faculté des Sciences et Techniques, 123 Avenue Albert Thomas, Université de Limoges,87060 Limoges, Franceb Pe@rl, ESTER Technopole, 20 rue Atlantis, 87068 Limoges Cedex, Francec LCSPBAT, UMR CNRS 7244, 74 Avenue Marcel Cachin, F-93017 Bobigny, France

a r t i c l e i n f o

Article history:Received 21 August 2013Accepted 19 January 2014Available online 27 January 2014

Keywords:Gold nanoparticlesBisphosphonatesUranyl ionSensingMaterial chemistry

a b s t r a c t

We describe a sensing system that relies upon the formation of specific complexes between uranyl ionand bisphosphonate-capped gold nanoparticles. The ensuing concentration-dependent red-shift ofsurface plasmon resonance absorption allows the determination of uranyl ion concentration withinthe 1.5–15 ppm range.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The development of colorimetric sensors capable of selectiverecognition of cationic species is a research area of current interest[1–2]. More particularly, sensing of uranyl ion is an importantchallenge for nuclear companies and environmental groups. Themanagement of uranium mines requires scheduled monitoring ofsurface and ground waters within the mining sites and in theirsurroundings. Determinations of uranium contents of water sam-ples are usually obtained through time-demanding radionuclideestimations carried out in remote places. These tasks could largelybenefit from the use of portable devices, preferably the ones basedon colorimetric methods which are certainly the most handywhen it comes to on-field analyses.

Techniques based on aggregation of gold nanoparticles (AuNPs) arecurrently knowing important advances [3] because they can lead toversatile applications dealing, for example, with sensing of oligonu-cleotides [4], metal cations [5–8], glucose [9], or the monitoringof enzyme activity [10]. These nanoparticles could also be used todevelop miniaturized electronic and optical devices [11]. AuNPs stabi-lized in water by a capping agent avoiding their aggregation exhibitsurface plasmon resonance properties, which are function of nano-particle size. Such suspensions exhibit a wine red color when the

particle diameter is comprised between 10 and 50 nm (λmax¼520 nm)[12]. Analyte complexation by the capping molecules elicits nanopar-ticle aggregation, and induces a red-shift of the surface plasmonresonance absorption peak; the solution turns purple to blue, depend-ing on analyte concentration. Bisphosphonates (BP) are known toform very stable and specific complexes such as tetra-, penta- or

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Materials Letters

UO22+

NRD:Au NPs Au NPs aggregation in presence of UO2

2+

Scheme 1. Schematic representation of the specific complexation of UO22þ by

bisphosphonate-capped AuNPs.

0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.matlet.2014.01.097

n Corresponding author: Tel.: þ33 5 5545 7490; fax: þ33 5 5545 7202.E-mail address: vincent.sol@unilim.fr (V. Sol).

Materials Letters 122 (2014) 208–211

hexadentate complexes with uranyl ions (UO2þ) [13–16]. So, in the

light of the above, we have synthesized bisphosphonate-capped goldnanoparticles which lead to the formation of complexes with uranylions, allowing the determination of uranyl ion concentration indistilled water within the 1.5–15 ppm range by UV–visible detection(Scheme 1).

2. Experimental

Chemicals and equipment: All solvents and reagents werepurchased from Aldrich, Prolabo or Acros. Analytical thin layerchromatography (TLC) was performed on Merck 60F254 silica gel.1H and 13C NMR spectroscopies were performed with a Brüker

P

OH

P

O OH

OH

O

OH

O-Na+

gold nanoparticles= AU NPs

i

ii

Neridronate : NRD NRD-AUNPs

iiiH2N(CH2)5COOHH2N

( )3

Scheme 2. Synthesis of neridronate-capped gold nanopaticles (NRD-AUNPs). (i) H3PO3, methanesulfonic acid, 65 1C, (ii) PCl3, 20 h then NaOH aqueous solution; and(iii) NaAuCl4, H2O, 50 1C then dialysis against distilled water during 24 h.

Fig. 1. (A) 1H NMR spectrum of the synthesized neridronate in D2O and (B) TEM image of NRD:AuNPs with an average diameter of about 27 nm (white bar¼50 nm).

N. Drogat et al. / Materials Letters 122 (2014) 208–211 209

DPX 400 spectrometer. Chemical shifts are reported as δ (ppm),downfield from internal TMS. Infra-red spectra were recorded on aPerkin Elmer Spectrum 1000 FTIR spectrometer with KBr pellets.ESI spectra were realized in positive mode with an Esqire 3000capture compound mass spectrometer (Brüker Daltonics, Bremen,Germany) equipped with an ESI source (Analytica Bradford, CT,USA). APCI spectra with ICOA (UMR 6005 CNRS) from the OrléansUniversity (France). Photomicrograph of AuNPs was obtained witha JEOL 2010 transmission electron microscopy (TEM) operated at400 kV accelarating voltage. UV–vis spectra were recorded on aSPECORDs 210 double beam spectrophotometer from AnalytikJena using 10 mm quartz cells.

Synthesis of neridronate (NRD): Neridronate (NRD) was chosen asthe capping agent. This molecule can bind AuNPs surface thanks toits primary amine group and also possesses a sufficiently longand flexible carbon chain [17–18]. NRD was prepared according tothe method proposed by Kieczykowski et al. [19]. To 2.5 g of ε-aminohexanoic acid (1 equiv.) in a three-necked round-bottom flask,H3PO3 (1.6 g, 1 equiv.) was added under inert atmosphere followedby 8 mL of methanesulfonic acid (Scheme 2). After heating at 65 1C,3.5 mL of PCl3 (2 equiv.) was slowly added (about 20 min) and thereaction was allowed to proceed for 20 h. The resulting yellowviscous reaction mixture was cooled to room temperature andquenched with 30 mL of ice-cold water. The pH was adjusted to4.3 with NaOH solution and the white precipitate formed wascollected by filtration, washed with EtOH and dried under vacuum,to give monosodium neridronate (43%). Physico-chemical datarelative to this compound are listed in Supporting information.

Synthesis of NRD-capped gold nanoparticles: 2.5 mL of a 5.2�10�4 M solution of NaAuCl4 and 2H2O were brought to 57.5 mL withdistilled water (Scheme 2). The temperature was raised with stirringto 50 1C, followed by addition of 2.5 mL of a 5.2�10�4 M solution ofneridronate. After 5 min the solution turned pink and no longerseemed to evolve. The final solution was dialyzed for 24 h (moredetailed protocol in electronic Supplementary material).

3. Results and discussion

Neridronate (NRD) has been characterized by 1H, 13C NMR inD2O (Fig. 1A), IR, and elemental analysis (electronic Supplementarymaterial). All data agreed with the literature [20]. Further mixingof NaAuCl4 with NRD dilute solutions resulted in the formation ofNRD-capped AuNPs with the requested size (Fig. 1) [21]. Afterdialysis against distilled water the NRD–AuNPs solution was setat 2.15�1010 AuNPs/mL, accounting for a NRD concentration of2.17�10�5 mol L�1. Fig. 1B shows a TEM micrograph of NRD:

AuNPs. Average particle size was 27 nm as calculated from particlehistogram.

Uranyl assay consisted in mixing equal volumes of uranyl acetateand NRD:AuNPs solutions. In presence of uranyl ion, a color changeoccurred in less than 10 min; the solution turned purple in presenceof 7.5 ppm of uranium (Fig. 2B) and blue with 15 ppm. This colorchange is demonstrated by the UV–vis spectra shown in Fig. 2A. Themaximum absorbance wavelength was shifted from 535 to 554 nmwhen using the 15 ppm solution. This red shift is due to theaggregation of the gold nanoparticles attesting uranyl ion complexa-tion with bisphosphonate [22].

Absorption maxima plotted against uranyl concentration led toa sigmoid titration curve (R²¼0.9987) (Fig. 3).

To assess the specificity of this method, nanoparticles were alsomixed with various metal cations (Fig. 4B) such as Naþ , Ca2þ ,Cd2þ , Cu2þ , Mn2þ , Zn2þ (acetates) in distilled water (neutral pH)with no color change. Similarly, the UV–vis spectra showed adramatic change with uranyl solution (7.5 ppm) compared to othercation samples with which no evolution of the maximum wave-length absorption was observed.

4. Conclusion

We propose a new method for the detection of uranyl ion with anew material: bisphosphonate-capped gold nanoparticles whosesynthesis is easy and quick. The use of these nanoparticles could bean attractive method because it does not require any sample prepara-tion and delivers results in minutes while it takes several days withcurrent methods (alpha spectrometry, gamma spectrometry). So, the

530

535

540

545

550

555

560

0 5 10 15 20

λmax

[UO22+] (ppm)

Fig. 3. Curve showing the correlation between uranyl concentration and λmax (E).

Fig. 2. (A) UV–vis spectra of NRD:AuNPs in presence of solutions containing 0 –15 ppm of UO22þ and (B) the corresponding solutions.

N. Drogat et al. / Materials Letters 122 (2014) 208–211210

efficiency, speed and ease of use of this method could be veryappropriate to detect and quantify uranyl anion in waste water.Further improvements are currently ongoing in our laboratory.

Acknowledgments

We acknowledge Dr. M. Guilloton for help in editing thismanuscript, Pierre Carles (SPCTS – Plate-forme de microscopieélectronique-CEC Limoges) for TEM analysis and the ‘ConseilRégional du Limousin’ for financial support.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.matlet.2014.01.097.

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Na Ca Cd Cu Mn Zn UO22+blank

0

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A

wavelength (nm)

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Mn

Ca

Cu

Zn

Na

U

Fig. 4. (A) UV–vis spectra of NRD:AuNPs in presence of solutions containing different metal ions concentrated at 7.5 ppm and (B) the corresponding solutions.

N. Drogat et al. / Materials Letters 122 (2014) 208–211 211