Page 2 - Surface-Confined Assemblies and Polymers for Molecular Logic

Surface-Confined Assemblies and Polymers de Ruiter and van der Boom

(Boolean) Logic and Computing (MBLC) offers a great degree represented by any system capable of existing in twoexclusive
of controllability and versatility over the intrinsic properties of states. The value of the binary digit (1 or 0) can be experimen-
3
the used material. Therefore, a chemist's bottom-up approach tally verified by assigning threshold values, with good ON/OFF
toward information processing might be an attractive ratios, to the output signature (e.g. absorbance or fluorescence)
alternative. 15 In this regard, molecular sensors perform a of the sensor; making it a suitable setup for MBLC.
form of information processing since they can detect changes
III. Molecular Combinatorial Logic Gates and
in their environment and transfer this information into an
Circuits
interpretable output. Thus, they often initiate the information
For logic gates with multiple inputs, the relationship be-
processingcascadeand displayclassical input/outputbehavior
tween their inputs and outputs can be described by algebraic
required for logic operations. Their inputs are binary in nature,
operations, as introduced by Boole. For two-input logic
for example, the absence (0) or presence (1) of an analyte. In
gates, this results in 16 distinct combinations, each repre-
addition, the output signature of molecule-based sensors
senting a specific logic gate. The NOR and NAND gates act as
(variation in the absorbance, fluorescence, etc.) is also binary
natural primitive logic gates, in the sense that all other logic
in nature, either absent or present. This elementary behavior 10
gates can be constructed from these gates. In practice only
can be described by the YES gate. This basic one-input logic
the AND, OR, and NOT gates are used for constructing logic
gate has an output that follows the nature of the input: present
circuits because of their simplicity and cost effectiveness.
or absent. Although they have limited application in digital
Our hexavalent chromium sensor only operates effi-
circuits, the observation that their behavior can be described in þ 9
ciently in the presence of H . In other words, only if H þ
binary terms led to the development of the first molecular logic
and Cr 6þ are present will the surface-confined polypyridyl
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gates. These principles were later extended to mimic more
complexes change their current state within a given time
complex logic circuitry and logic gates. In this Account, the use
period. The output at λ = 516 nm is equivalent to a logic NAND
of surface-confined polypyridyl complexes of osmium and
gate, whereboth inputs have tobe high(1) in order to generate
ruthenium and poly(3,4-ethylenedioxythiophene) (PEDOT)
a negative (0) output (the initial state here is 1). According to De
for MBLC will be discussed (Scheme 1). Morgan's theorem, the AND gate is created if one chooses
the absorption band at λ = 317 nm, since this is complemen-
II. Molecular Sensors
tary to the absorption band at λ = 516 nm. By changing the
Our group has used surface-confined monolayers of os- nature of the chemical inputs to, for example, a combination
mium and ruthenium complexes (Scheme 1A) for the selec- of CoCp 2 and H 2 O, a NOR logic gate was constructed, while
þ
tive detection and quantification of H 2 O, NO ,Fe 2þ/3þ ,Cr 6þ , utilizing the same Os 2þ/3þ -based monolayer. In a similar
and NO x in various matrixes (i.e., water, organic solvents, manner, using different combinations of inputs, the formation
air). 7,8 The basis for this robust sensory platform is the transfer of the AND, XOR and the INHIBIT gates were demonstrated,
of electrons between the immobilized metal complexes and and by doing so, their complementary counterparts: NAND,
the analyte. The subsequent change in the formal oxidation NOR, and NXOR as well, according to De Morgan's theorem. 11
state of the osmium complex (from M 2þ to M 3þ or vice versa) Figure 2 shows one of the first examples that demon-
is accompanied by a significant reduction in the metal-to- strated that monolayers could be used for constructing logic
ligand charge-transfer (MLCT) band at λ max ≈ 500 nm and gates/circuits that processed information provided by che-
the ligand-to-metal charge-transfer (LMCT) band at λ = 317 mical inputs. 11 However, predesigning a desirable logic
nm (Figure 1). These properties have been used for devel- function or to introduce proof-of-principles, as shown re-
oping a new method for detecting part-per-million (ppm) cently in molecular gaming, photodynamic therapy, and/or
levels of highly toxic Cr 6þ in water. 9 This test can be medical diagnostics, is a major challenge. 1215 A timeline
performed in as little as 1 min and can be used in natural is displayed in Figure 3 that gives a noncomprehensive
systems. The molecular-based sensor is reset to its original overview when some concepts were introduced. From
state by washing with water. It is highly selective and stable 1993 to 2000, combinatorial logic developed, and
under various conditions. Moreover, dual sensing was demon- molecular logic gates and elementary arithmetic were
strated with the same monolayer since Fe 3þ can be detected demonstrated. 16,17 From 2007 onward, sophisticated logic
under neutral conditions, whereas the detection of Cr 6þ re- circuits were designed, as in keypad locks, 18 de-/encoders, 19
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quires acidic conditions. Since a binary number can be (de)multiplexers, 20 and full adders/subtractors. 21,22 These

564 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 563–573 ’ 2011 ’ Vol. 44, No. 8