Angewandte
Chemie
results, we have designed a logic device that defines a
threshold of pH and HgII as well as Clꢀ or Brꢀ ions as inputs,
and the fluorescence signal of 1 as output. For input, the
presence and absence of HgII ions (> 6 ꢀ 10ꢀ5 m) and Clꢀ or
Brꢀ ions (with 2 equivalents of HgII added) is defined as 1 and
0, respectively, and pH values greater than 4 and less than 4 as
0 and 1, respectively. For output, we define the normal
fluorescence of 1 as 1 and the quenched fluorescence as 0
(detailed in Figure 3). Based on the above definitions and the
fluorescence intensity at 530 nm, binary transfer of a logic
operation can be realized by controlling the three inputs of
pH and HgII as well as Clꢀ or Brꢀ ions, and by monitoring the
fluorescence output from 1. The truth table and a schematic
representation of the logic gates are presented in Table 1.
ꢀ5
ꢀ5
~
&
*
Figure 4. Titration of 3 ( 1.0ꢀ10 m), 2 ( 1.0ꢀ10 m), and 1 (
48 mgmLꢀ1) with HgII in aqueous solution containing 4% CH3CN
(lex =330 nm).
Table 1: Truth table for the logic gate 1. The values in parentheses in the
output column indicate the experimental fluorescence intensities in
arbitrary units. The corresponding binary states are determined by
applying a threshold value of IF =200. The errors were determined by
repeating these experiments seven times.
the fluorescence of 3, so as to assure maximum quenching,
since more HgII ions are required to quench the fluorescence
of 1 than that of 3 (Figure 4).
By titration of the 3–HgII and 1–HgII systems with Clꢀ,
Brꢀ, or Iꢀ ions, we found that the fluorescence of 3 and 1
gradually recovered and reached a point of inflexion at a
concentration ratio Clꢀ, Brꢀ, or Iꢀ/HgII ratio of 2:1 (Figure S8
in the Supporting Information). The titration experiments of
3–HgII also showed that the addition of Clꢀ, Brꢀ, or Iꢀ ions
caused precipitation of HgCl2, HgBr2 , or HgI2. The amount of
visible precipitation is in accordance with the solubility of
these HgII halides. In contrast to the addition of Clꢀ or Brꢀ
ions, addition of a small quantity of Iꢀ ions caused the
formation of a heavy yellow precipitate, which complicated
the fluorescence measurement. The yellow precipitate was
identified as the beta form of HgI2 by using X-ray diffraction.
By monitoring the fluorescence intensity of 3, we determined
that more than 83% of the fluorescence of 3 could be
recovered by addition of iodide ions after centrifugation and
removal of HgI2; the remaining fluorescence quenching may
be attributed to the loss of 3 that arises from its adsorption to
HgI2. On the other hand, a 1H NMR titration with 3
(Figure S9 in the Supporting Information) also indicated
that the addition of Clꢀ ions led to the effective recovery of
the original 1H NMR spectrum of 3 after the changes caused
by the addition of HgII ions. These results demonstrated that
the interaction between HgII and Clꢀ, Brꢀ, or Iꢀ ions is
stronger than that between HgII ions and 3 or 1. As a result,
HgX2 was formed, which led to the recovery of the
fluorescence signal of 3 and 1. Therefore, for logic gate
operation, it is more desirable to use Clꢀ or Brꢀ than Iꢀ ions,
as the precipitation of HgI2 complicates the detection and
reduces the fluorescence intensity. Two equilibrium reactions
exist in this system:
Input 1
pH
Input 2
HgII
Input 3
Output
F1 (lem =530 nm)
Clꢀ or Brꢀ
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
1 (332ꢁ12)
1 (342ꢁ14)
0 (134ꢁ8)
1 (308ꢁ17)
0 (130ꢁ8)
0 (129ꢁ10)
0 (76ꢁ5)
0 (111ꢁ6)
To develop a better understanding of the mechanisms of
the logic gate, the dependence of the fluorescence of 3 on
metal ions and anions was further investigated. Many differ-
ent optical applications use 3 as fluorophore when it is linked
to a receptor.[27–30] Experimental results suggest that the
dependence of the fluorescence intensity of 3 in the presence
of various ions is similar to that of 1 (Figures S4–S6 in the
Supporting Information). HgII ions effectively quench the
fluorescence of 3, whereas other metal ions cause a negligible
fluorescence change. The fact that PbII and Iꢀ ions do not
quench the fluorescence of 3 (Figure S7 in the Supporting
Information) suggests that the quenching action of HgII ions is
unlikely to be associated with the heavy atom effect. Rather,
the fluorescence quenching may be attributed to charge
transfer within 3 and HgII ions.[27,28] One can estimate the
minimum ratio of HgII ions to 3 to achieve maximum
quenching from the titration data between HgII ions and 3.
The experimental data (Figure 4) indicate that a HgII/3 ratio
of about 12 was required for this purpose. For logic gate
operation with 1, 70% more HgII ions were employed than
the minimum amount required for maximum quenching of
HgII þ DA ꢀ SiNWsðorDAÞ Ð DA ꢀ SiNWsðorDAÞ ꢀ HgII
HgII þ 2 Xꢀ Ð HgX2
Angew. Chem. Int. Ed. 2009, 48, 3469 –3472
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3471