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Sensors and Actuators B 144 (2010) 88–91

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

ND molecular logic using semiconductor quantum dots

avneet Kaura, Narinder Singha, Bridgeen McCaughana, John F. Callanb,∗

School of Pharmacy and Life Sciences, The Robert Gordon University Aberdeen, Scotland AB10 1FR, UKDepartment of Pharmacy and Pharmaceutical Sciences, School of Biomedical Sciences, University of Ulster, Northern Ireland BT52 1SA, UK

r t i c l e i n f o

rticle history:eceived 12 August 2009ccepted 11 September 2009

a b s t r a c t

CdSe/ZnS quantum dots (QD) have been surface functionalised with mercaptoaniline and N-(o-methoxyphenyl)aza-15-crown-5 receptors, the former serving as a receptor for protons with the latterserving as a receptor for sodium ions. In conditions of low protons and sodium ions the QD fluorescence

vailable online 21 October 2009

eywords:uantum dotshotoinduced electron transferolecular logic

was quenched by a photoinduced electron transfer (PET) process from the receptors to the excited QD.The simultaneous addition of both protons and sodium ions, however, restored the QD emission whilethe independent addition of either ion had no effect. Thus, the conditions of a two input AND molecularlogic gate were satified with protons and sodium ions as inputs and 560 nm QD fluorescence as output.

© 2009 Elsevier B.V. All rights reserved.

odiumrotons

. Introduction

Boolean logic is an algebraic system of logic that forms the basisf today’s information technology (IT) systems. These electronicevices function on logic gates created from bulk semiconduc-ors that employ electronic (e.g. voltage) input and output signals.lthough a semiconductor feature can now be downsized to 32 nm

1], it will prove difficult to approach the dimensions of a singleolecule. The concept of molecular logic, pioneered by de Silva

nd co-workers in the early 1990s [2] led to supramolecular con-tructs capable of functioning as a range of logic gate types (i.e.R, AND, INHIBIT, etc.) [3]. In contrast to bulk semiconductors

hese molecular logic gates comprised of chemical inputs and auorescence output. Although bulk semiconductors are typicallyon-fluorescent, semiconductor quantum dots (QDs) do emit flu-rescence when excited, due to their small size (2–10 nm) and ahenomenon known as quantum confinement [4]. Due to their

mpressive photophysical properties, QDs have been widely stud-ed as both biochemical tags and optical sensors in recent years [5].owever, there have been no reports of their use in logic gate oper-tions. Here, we present the first example of a semiconductor QDased AND molecular logic gate with Na+ and H+ ions as inputs and

60 nm fluorescence emission as output.

We have previously demonstrated that the fluorescence emis-ion of QDs can be selectively switched “off” by appending ionpecific organic receptors to their surface [6]. Binding of the target

∗ Corresponding author. Tel.: +44 02870323059; fax: +44 02870324965.E-mail address: j.callan@ulster.ac.uk (J.F. Callan).

925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2009.09.066

analyte to the receptor resulted in a restoration of the QD fluo-rescence. This “off–on” fluorescence response was shown to becontrolled by modulating the rate of photoinduced electron trans-fer (PET) between the receptor and the excited QD. Here, we extendthe principle by anchoring two PET active receptors to the QDsurface, one an aniline unit to bind H+ ions and the second a N-(o-methoxyphenyl)aza-15-crown-5 unit selective for Na+ ions. Thepresence of two PET channels should require both H+ and Na+ to bepresent simultaneously to arrest the PET process and switch “on”QD emission. Thus, the conditions of an AND logic gate are satisfied.

2. Experimental

2.1. Synthesis of 2–4 and 6, 7

2.1.1. Materials and reagentsChemicals were purchased from Aldrich Co., and used as

received without further purification. Green emitting CdSe–ZnSQDs were purchased from Evident Technologies, New York (prod-uct No. ED-C10-TOL-0545).

2.1.2. Equipment and parametersUV–vis measurements were recorded on an Agilent UV–Vis

Spectrometer using 10 mm quartz cuvettes. Fluorescence mea-surements were recorded on a Perkin Elmer LS55 Luminescence

Spectrometer using 10 mm quartz cuvettes. Excitation wavelengthunless otherwise stated was set at 370 nm. Excitation slit size was10.0 nm and emission slit size was 10.0 nm. Scan speed was set at500. NMR spectra were recorded on a Bruker Ultrasheild 400 MHz.Chemical shifts are reported in parts per million (ı) downfield of

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N. Kaur et al. / Sensors and

MS. Particle size distributions were recorded on a Malvern NanoZSnstrument at 25 ◦C in a 10 mm cuvette using a He–Ne laser of33 nm. The average size is the average of 20 independent experi-ents and the standard deviation is taken as the error.

.1.3. N,N-bis(2-hydroxylethyl)-2-methoxyaniline (2) [9]A solution of 5 g (40 mmol) of 1 in 2-chloro-ethanol (32.5 mL,

.49 mol) and 7 mL of water was heated to 80 ◦C for 15 min. K2CO317 g, 0.12 mol) was slowly added such that the temperature ofhis exothermic reaction was kept below 110 ◦C. The mixture waseated at 95 ◦C for 22 h, cooled, and unreacted 2-chloroethanolas removed under vacuum. The residue was diluted with water

100 mL) and extracted with CHCl3 (3× 100 mL). The CHCl3 solu-ion was dried over MgSO4, and the solvent was evaporated tofford 7.8 g (92%) of light brown oil. 1H NMR (400 MHz, CDCl3) ı.17 (br, 2H, OH), 3.21 (t, 4H, N–CH2), 3.50 (t, 4H, O–CH2), 3.89s, 3H, ArOCH3), 6.93–7.23 (m, 4H, ArH). MS (MeOH, ES-API) m/zxpected: 211.1, found: 212.1 (M+H).

.1.4. 2-Methoxyphenylaza-15-crown-5 (3) [7]Formation of the crown ether was accomplished as follows. A

50 mL three-necked, N2-flushed flask was purged with N2. NaH60% in oil, 2.98 g, 74.5 mmol) was added to the reaction vessel andashed with hexanes (4× 100 mL). THF (80 mL) was then added

o the flask. This suspension was heated to reflux with vigoroustirring. A solution of 2 (7.5 g, 35.5 mmol) and triethylene glycolitosylate (16.2 g, 35.5 mmol) in THF (30 mL) was added dropwise.eflux was continued for 20 h. The reaction mixture was cooled anduenched with H2O, and the solvent was evaporated in vacuo. Theesidue was dissolved in H2O (100 mL) which was extracted withH2CI2 (3× 100 mL). The combined organic layers were reducedo a minimum volume. The crude mixture was chromatographedAl2O3, 0–2% 2-PrOH/hexanes) to yield crown ether 3 (4.73 g, 41%)s a pale yellow oil. 1H NMR (400 MHz, CDCl3) ı 3.48 (t, 4H, N–CH2),.68 (m, 16H, O–CH2), 3.83 (s, 3H, ArOCH3), 6.83–6.96 (m, 3H, ArH),.13 (dd, 1H, ArH). MS (MeOH, ES-API) m/z expected: 325.19, found:26.2 (M+H).

.1.5. N-(o-methoxyphenyl)aza-15-crown-5 (4)This product was prepared with the modified literature method

9]. To an oven-dried flask equipped with a magnetic stir bar wasdded 1.5 mL anhydrous DMF. The flask was flushed with N2 and

cheme 1. Synthesis of 7. (i) ClCH2CH2OH, K2CO3, H2O; (ii) TsOCH2(CH2OCH2)2CH2OTs, N

ators B 144 (2010) 88–91 89

cooled in an ice-water bath. To the stirring DMF at 0 ◦C was addeddropwise POCl3 (0.82 mL, 9 mmol). After stirring for 10 min, 3 (1 g,3 mmol) was added dropwise as a solution in 1.5 mL anhydrousDMF. The resulting yellow solution was allowed to warm at roomtemperature. After stirring at room temperature for 16 h, the solu-tion was heated to 80 ◦C for 1 h, cooled, and poured into 50 g ofice; the flask was rinsed with 5 mL of water; and the combinedaqueous solutions were adjusted to pH 7 (by pH paper) with satu-rated K2CO3. The solution was extracted with CHCl3 (3× 50 mL), theCHCl3 phase washed with water (3× 50 mL) then dried over NaSO4,filtered and concentrated in vacuo. The crude brown oil was puri-fied by flash chromatography (Al2O3, 0–10% 2-PrOH/hexanes) togive 4 as light orange crystals (500 mg, 47%). 1H NMR (400 MHz,CDCl3) ı 3.64 (m, 16H, O–CH2), 3.72 (t, 4H, N–CH2), 3.81 (s, 3H,ArOCH3), 6.99 (d, 1H, ArH), 7.31–7.36 (m, 2H, ArH), 9.75 (s, 1H,–CHO). MS (MeOH, ES-API) m/z expected: 353.1, found: 354.1(M+H).

2.1.6. Synthesis of MA-capped CdSe–ZnS nanoparticles (6)The MA-capped CdSe–ZnS nanoparticles were prepared with

the ligand exchange reaction developed by Tomasulo et al. [8]. Asolution of CdSe/ZnS core based QDs (0.5 mL, 27 nmol) was addedto 2-mercaptoaniline (MA, 1.25 g, 0.01 mol) in dry chloroform.The reaction was allowed to reflux for 18 h. Upon completion ofreaction, the solvent was removed under reduced pressure. Thecrude mass was suspended in acetonitrile (5 mL) and centrifugedat 12,500 rpm for 5 min. The supernatant solution was decanted offand the solid was again suspended in fresh acetonitrile. This stepwas repeated twice and the product was vacuum dried to obtainpure 6 as a yellow coloured powder. 1H NMR (400 MHz, DMSO-d6)ı 6.79 (m, 1H, ArH), 7.01 (m, 1H, ArH), 7.32 (m, 1H, ArH).

2.1.7. Synthesis of QD–MA–azamacrocycle conjugate (7)The QD–MA–azamacrocycle conjugates were prepared by mix-

ing MA-capped QDs, 100 mg (0.3 mmol) of 4 in 10 mL of anhydrousMeOH and refluxed for 24 h. Upon completion of reaction, the sol-vent was removed under reduced pressure. The crude mass was

suspended in acetonitrile (5 mL) and centrifuged at 12,500 rpm for5 min. The supernatant solution was decanted off and the solid wasagain suspended in fresh acetonitrile. This step was repeated twiceand the product was vacuum dried to obtain pure 7 as an orangecoloured powder.

aH, THF; (iii) POCl3, DMF; (iv) 2-aminothiophenol, CHCl3, reflux; (v) MeOH, reflux.

90 N. Kaur et al. / Sensors and Actuators B 144 (2010) 88–91

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Fig. 2. (a) Fluorescence spectra of 7 in the presence of: (i) low H+, low Na+ (redline); (ii) high H+, low Na+ (pink line); (iii) low H+, high Na+ (green line); and(iv) high H+ and high Na+ (blue line). (b) Truth table for AND logic behaviourof 7. [7] = 1.05 × 10−7 M, solvent CH3CN:H2O (98:2, v/v). Low H+ = 10−8.7 M; high

+ −6.2 + + −3.0

neous addition of high H (10 M) and Na (10 M) ions as

Fig. 1. 1H NMR spectrum of 7 recorded in d6-DMSO.

. Results and discussion

The probe was prepared by following the synthetic stepsutlined in Scheme 1. o-Anisidine (1) was dialkylated with-chloroethanol [9] and then reacted with triethylene glycolitosylate and NaH in refluxing THF to afford the N-(o-ethoxyphenyl)aza-15-crown-5 3 [7]. Upon formylation of 3 the

ldehyde 4 was obtained in a moderate yield.The most common method of attaching organic ligands to

he surface of preformed QDs is through ligand exchange, whichnvolves replacement of the original trioctylphosphine oxideTOPO) ligands for thiol terminated ligands [10]. However, this

ethod normally involves the use of a base such as tetrabutylam-onium hydroxide to generate the thiolate anion. The byproduct of

his reaction is the tetrabutylammonium salt which is problematiciven its propensity to complex with azo crown ether. This com-lexation may lead to the undesired cancellation of PET from therown ether receptor in the absence of Na+. Therefore an alterna-ive novel method of attaching this receptor to the QD was adopted.he native TOPO groups of CdSe/ZnS QDs were first exchanged with-mercaptoaniline to produce QDs decorated with amine function-lity (6). 1H NMR showed this exchange to be successful with theethylene and methyl protons of the TOPO groups, present in the

pectrum of the native QDs being absent in the spectrum of 6 (seeig. S7). The aldehyde 4 was then attached to 6 via a facile Schiffase forming condensation reaction resulting in compound 7.

Fig. 1 shows the 1H NMR spectrum of 7 and proves attachmentf 4 to the QD through Schiff base formation. There was no evidencef the aldehyde proton of 4 present in the spectrum of 7 (see Fig. S8)ndicating all the available 4 was consumed during the reaction. Thextent of attachment was judged to be 20% by relative integrationf the doublet centered at 7.15 ppm and the singlet at 3.75 ppm,epresenting one of the aromatic protons of the unreacted anilinenits and the methoxy protons respectively. This low level of incor-oration was also evidenced by the range of shifts in the aromaticegion due to the three distinct aromatic units, with those from thenreacted aniline unit being more intense than those from the tworidging the imine group. There was also a marked upfield shift ofhe azo crown N–CH2 protons from 3.71 ppm in 4 to 3.25 ppm in, indicating these protons are in a more shielded environment asresult of the increased conjugation after Schiff base formation.

hus, Schiff base formation provides an alternative strategy for the

urface decoration of QDs with organic ligands. The average hydro-ynamic diameter observed for 7 was determined as 10.05 nm byynamic light scattering, slightly larger than that of 6 (9.33 nm) (seeigs. S9 and S10).

H = 10 M; low Na = 0 M; high Na = 10 M. (For interpretation of the refer-ences to color in this figure legend, the reader is referred to the web version of thearticle.)

The photophysical properties of 6 and 7 were investigatedin a ACN:H2O (98:2, v/v) solution. The UV–vis spectra of boththese compounds were dominated by the absorbance from theappended organic ligands but both also showed evidence of theQD first exciton peak at �MAX 530 nm (see Fig. S11). When 6 wasexcited at 415 nm there was no evidence of QD emission at �MAX560 nm, which was attributed to PET from the aniline nitrogen tothe excited QD quenching fluorescence by non-radiative decay.As expected, attachment of receptor 4 onto the surface of theQD had no effect on the emission of the QD with the fluores-cence spectrum of 7 also showing no emission at 560 nm. Theincorporation of the N-(o-methoxyphenyl)aza-15-crown-5 unitintroduces a second PET channel within 7 that can also lead toQD fluorescence quenching [9]. Therefore, 7 contains two dis-tinct PET channels that must be blocked before QD fluorescenceis observed.

To investigate if the fluorescence of 7 could be recovered bycancelling these two PET channels input conditions of high H+

(10−6.2 M) and high Na+ (10−3.0 M) were examined. Fig. 2 showsthat the independent addition of either H+ or Na+ resulted in onlyminor changes of the QD emission at �MAX 560 nm. There washowever an enhancement in the band at �MAX 505 nm upon Na+

addition which was attributed to emission from the Schiff baseluminophore [11]. This enhancement can be explained by the factthat when Na+ binds to the N-(o-methoxyphenyl)aza-15-crown-5receptor the methoxy unit serves to cap the Na+ ion to increasethe number of oxygen’s in the co-ordination sphere [12]. Thiscapping prevents isomerisation of the C N bond that otherwiseleads to non-radiative decay of the �MAX 505 band [13]. However,QD emission (�MAX 560 nm) was only restored upon the simulta-

+ −6.2 + −3.0

shown in Fig. 2a. Thus both PET channels must become blockedwhen Na+ and H+ ions are present in these quantities. But whydoes QD fluorescence not switch “on” in the high H+ input stategiven that both receptors contain basic nitrogen atoms? It has

N. Kaur et al. / Sensors and Actu

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ig. 3. Changes in fluorescence spectra of 6 at pH 8.7 (purple line) and upon additionf H+ (pH = 6.2, blue line). Shown in green is the overlaid spectrum of the CdSe/ZnSarent QDs. Solvent = CH3CN/H2O (98:2, v/v). (For interpretation of the referenceso color in this figure legend, the reader is referred to the web version of the article.)

een shown that while in aqueous solution tertiary amines areonsidered stronger bases than primary amines there is no discern-ble difference in their pKa values in acetonitrile [14]. Therefore,e believe that at a concentration of 10−6.2 M H+ ions there are

nsufficient H+ ions to protonate all the available amine nitrogen’s.he literature data reveals the association constant between N-(o-ethoxyphenyl)aza-15-crown-5 and Na+ to be 10−3.9 M [7], and

ven accounting for some electronic differences due to increasedonjugation through Schiff base formation it should be well satu-ated when Na+ is present at 10−3.0 M. This Na+ chelation by the-(o-methoxyphenyl)aza-15-crown-5 receptor has been shown

o produce a similar fluorescence enhancement as H+ ions in allrganic PET sensors containing this receptor. Therefore, we believehat when Na+ and H+ inputs are high, Na+ ions preferentiallyccupy the crown ether receptor meaning there is now sufficient+ ions to protonate the remaining primary amine sites resulting

n the observed restoration in the QD fluorescence. As a control weubjected 6 to conditions of high pH (10−6.2 M H+) and observednly a minor recovery of the QD fluorescence (Fig. 3). We wererecluded from increasing the H+ ion concentration further dueo the instability of thiol capped QDs at low pH. However, weelieve again that this minor enhancement of QD fluorescencehows that there is insufficient H+ ions to protonate all the primarymines.

In conclusion, we have developed a two input AND molecularogic gate from semiconductor QDs. This gate functions using H+

nd Na+ ions as inputs and 560 nm QD fluorescence as an output.o the best of our knowledge, this is the first reported molecularogic gate using semiconductor QDs.

cknowledgements

The authors would like to acknowledge financial assistance fromhe EPSRC and RGU.

ators B 144 (2010) 88–91 91

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.snb.2009.09.066.

References

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(2008) 2468;(b) A.P. de Silva, S. Uchiyama, Nature Nanotechnol. 2 (2007) 399;(c) F.M. Raymo, S. Giordani, J. Am. Chem. Soc. 124 (2002) 2004;(d) F.M. Raymo, R.J. Alvarado, S. Giordani, M.A. Cejas, J. Am. Chem. Soc. 125(2003) 2361;(e) H. Tian, B. Quin, R.X. Yao, X.L. Zhao, S.J. Yang, Adv. Mater. 15 (2003) 2104;(f) H.M. Wang, D.Q. Zhang, X.F. Guo, L.Y. Zhu, Z.G. Shuai, D.B. Zhu, Chem. Com-mun. (2004) 670;(g) S. Uchiyama, G.D. Mc Clean, K. Iwai, A.P. de Silva, J. Am. Chem. Soc. 127(2005) 8920;(h) Y. Liu, W. Jiang, H.Y. Zhang, C.J. Li, J. Phys. Chem. B. 110 (2006) 14231;(i) D.D. Magri, T.P. Vance, A.P. de Silva, Inorg. Chim. Acta 360 (2007) 751.

[4] K.E. Anderson, C.Y. Fong, W.E. Pickett, J. Non-Cryst. Solids 299–302 (2002) 1105.[5] (a) K.M. Gattas-Asfura, R.M. Leblanc, Chem. Commun. (2003) 2684;

(b) Y. Cheng, K.M. Gattas-Asfura, V. Konka, R.M. Leblanc, Chem. Commun.(2002) 2350;(c) M.G. Sandros, V. Shete, D.E. Benson, Analyst 131 (2006) 229;(d) M.G. Sandros, D. Gao, D.E. Benson, J. Am. Chem. Soc. 127 (2005) 12198;(e) S. Banerjee, S. Kar, S. Santra, Chem. Commun. (2008) 3037;(f) J.F. Callan, R.C. Mulrooney, S. Kamila, B. McCaughan, J. Fluoresc. 18 (2008)527;(g) Y. Chen, Z. Rosenzweig, Anal. Chem. 74 (2002) 5132.

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Biographies

Dr. Navneet Kaur received her early education in India followed by a postdoctoralsecondment in South Korea. She joined the research group of Dr. Callan in 2008 asa Research Fellow working on the design of PET active receptors for conjugation toquantum dots.

Dr. Narinder Singh received his early education in India followed by postdoctoralsecondments in Japan and South Korea. He joined the research group of Dr. Callan in2008 working on an EPSRC funded project on multianalyte sensing with luminescentquantum dots.

Dr. Bridgeen McCaughan received her early education at Queens University Belfastfollowed by a postdoctoral secondment at RGU where she is now a lecturer in medic-inal chemistry. Her research interests include supramolecular photochemistry andphotodynamic therapy.

Dr. John Callan was educated at Queens University Belfast which was followed byindustrial secondments in the Pharma industry. Following a research fellowship atQUB he joined RGU as a lecturer in medicinal chemistry in 2004. He moved to theUniversity of Ulster in Northern Ireland in 2009 where he continues his researchinto the development of novel fluorescent sensors, quantum dots and photodynamictherapy.