RSC Advances
Paper
a
Table 2 Association constants (K ), limits of detection and semi-empirical energy values of the receptors with UA
ꢁ1
ꢁ1
Complex
UV-vis method (K
a
M
)
Fluorescence method (K
a
M
)
Limit of Detection (M)
Semi-empirical energy (a.u.)
4
4
ꢁ8
Complex R1C
Complex R2C
Complex R3B-C
Complex R4A-C
0.996 ꢂ 10
0.188 ꢂ 10
19.5121 ꢂ 10
0.1089
0.0083
0.0950
0.1369
4
4
ꢁ8
6.114 ꢂ 10
1.767 ꢂ 10
9.0609 ꢂ 10
4
4
ꢁ8
3.542 ꢂ 10
1.279 ꢂ 10
9.7194 ꢂ 10
4
4
ꢁ8
0.158 ꢂ 10
0.319 ꢂ 10
9.9682 ꢂ 10
the UV-vis experiments. When R2 was excited at 351 nm, the uorosensors are very important issues from medicinal and
emission spectrum had a peak maxima at 409 nm. Signicant clinical points of view.
quenching of the uorescence intensity was observed upon the
gradual addition of UA (upto 3 equiv.) (Fig. 9B). The uores-
cence quenching, which is due to a considerable modication
Acknowledgements
of the excited state of the receptor, obviously conrms the
complex formation between R2 and UA. The most probable
mechanism for the uorescence quenching effect involves an
inversion between the strongly emissive p–p* and the poorly
SD acknowledges DST (Govt. of India) for awarding him the
DST-FAST Track young scientist project (SR/S1/CS-108/2009). DS
is sincerely thankful to Indian School of Mines for the junior
research fellowship.
emissive n–p* states of the uorophore. Such a uorescence
quenching occurs due to hydrogen bonding interactions
between the receptor and UA, which leads to a greater stabili-
zation of the n–p* state with respect to the p–p* state and
Notes and references
1
2
3
(a) M. M. Conn and J. Rebek, Jr, Chem. Rev., 1997, 97, 1647;
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(a) C. Schmuck and W. Wienand, Angew. Chem., Int. Ed.,
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Abstr., 1992, 116, 235628p; (d) Chem. Abstr., 1991, 114,
results in a signicant decrease in the uorescence emission
(
2
16
intensity. Signicant uorescence quenching effects were
observed for the other receptors too and the uorescence
spectra of these receptors are in the ESI.†
2
Fig. 10 shows the actual stoichiometries of the complexes
formed during the UV-vis titrations of the receptors (R2, R3
and R4) with UA in water. When we plotted the concentration
ratio of guest to host ([G]/[H]) against the intensity change in
the UV-vis absorbance (DI), the curve, aer [G]/[H] ¼ 1, was
parallel to the x-axis for R2, implying that the complex formed
in 1 : 1 ratio (R2C). Initially the intensity of the absorption
changed sharply, then very slowly (Fig. 10). No such clear
break in the titration curves was observed for the other
receptors R3 and R4 with UA as there were weaker interactions
compared to R2 with UA.
8
1808s; (e) C. Cotrel, C. Guyon, G. Roussel and G. Taurand,
Eur. Pat. Appl., EP 208621 1987; (f) FR Appl. 85/10619, 1985;
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(
4
5
(a) P. S. Corbin, S. C. Zimmerman, P. A. Thiessen,
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123, 10475; (b) T. R. Kelly, G. J. Bridger and C. Zhao, J. Am.
Chem. Soc., 1990, 112, 8024.
The association constants (K
a
) were calculated from linear
and are summarised in Table 2.
values with the semi-empirical energy values
(a) C. J. Chandler, L. W. Deady, J. A. Reiss and V. Tzimos, J.
Heterocycl. Chem., 1982, 19, 1017; (b) C. J. Chandler,
L. W. Deady and J. A. Reiss, J. Heterocycl. Chem., 1981, 18,
17
regression analysis
Comparing the K
a
calculated theoretically, it is clear that R2 and R3 bind to UA
more strongly than the other two receptors R1 and R4. The self-
complementary nature of R1 and the strong intra-molecular H-
599; (c) S. P. Goswami, R. Mukherjee, R. Mukherjee,
S. Jana, A. C. Maity and A. K. Adak, Molecules, 2005, 10, 929.
T. R. Kelly and M. P. Maguire, J. Am. Chem. Soc., 1987, 109,
6
a
bond formation in R4 result in lower K values compared to R2
6549.
and R3. The limits of detection (LOD) estimated by the uo-
rescence method also indicated that among all four receptors,
R2 can sense the lowest concentration of UA (Table 2).
7
8
R. K. Bera and C. R. Raj, Chem. Commun., 2011, 47, 11498.
(a) V. J. Pileggi, J. D. Giorgio and D. K. Wybenga, Clin. Chim.
Acta, 1972, 37, 141; (b) G. F. Domagk and H. H. Schlicke,
Anal. Biochem., 1968, 22, 219.
9
(a) R. A. Poole, F. Kielar, S. L. Richardson, P. A. Stenson and
D. Parker, Chem. Commun., 2006, 4084; (b) M. D. Cowart,
I. Sucholeiki, R. R. Bukownik and C. S. Wilcox, J. Am. Chem.
Soc., 1988, 110, 6204; (c) E. Klein, M. P. Crump and A. P. Davis,
Angew. Chem., Int. Ed., 2005, 44, 298; (d) P. Rzepecki and
T. Schrader, J. Am. Chem. Soc., 2005, 127, 3016; (e) T. Grawe,
G. Schafer and T. Schrader, Org. Lett., 2003, 5, 1641.
Conclusions
This work was predominately focused upon the development of
host–guest systems that are able to form an array of donor and
acceptor atoms. Naphthyridine based water soluble receptors
(R1–R4) are in fact, procient in binding to UA in water. These
receptors are signicant for their use as uorogenic sensors.
The detection and estimation of UA in water using synthesised 10 R. P. Sijbesma and E. W. Meijer, Chem. Commun., 2003, 5.
432 | RSC Adv., 2014, 4, 428–433
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