Resettable, multi-readout logic gates based on controllably reversible aggregation of gold nanoparticles

Article abstract of DOI:10.1002/anie.201008198

A resettable logic system based on spiropyran-modified gold nanoparticles that is capable of AND, OR, and INHIBIT logic operations has been constructed. Several methods can record the output of this process, including the naked eye, UV/Vis spectroscopy, d

Full text of DOI:10.1002/anie.201008198

DOI: 10.1002/anie.201008198
Logic Gates
Resettable, Multi-Readout Logic Gates Based on Controllably
Reversible Aggregation of Gold Nanoparticles**
Dingbin Liu, Wenwen Chen, Kang Sun, Ke Deng, Wei Zhang, Zhuo Wang,* and Xingyu Jiang*
In recent years, more and more chemical systems have been
employed to perform Boolean logic operations, resulting in
the remarkable progress of various molecule-based logic
systems, such as AND, OR, XOR, NAND, NOR, INHIBIT,
half-adder, and half-subtractor.[1] The set–reset function in
molecular digital systems is an important step to construct
memory elements of sequential logic operations that have the
possibility to store information in a write–read–erasable
form.[2] Most of reported molecular logic gates have the
following disadvantages: 1) they are not resettable; and
2) they employ small molecules or biomacromolecules as
inputs and a fluorescent signal as the output, which relies on
advanced instruments for the readout.[3] Gold nanoparticles
(AuNPs) have recently attracted considerable attention
because of their many distinctive physical and optical proper-
ties. In particular, the surface plasmon resonance (SPR) of
AuNPs can be changed by modulating the distance between
AuNPs, along with the color change of AuNPs solutions.[4]
Therefore, by using AuNPs, the molecular events in logic
systems can be easily transformed into color changes, which
can be monitored by the naked eye. Moreover, the change of
dispersion states of AuNPs can be further monitored by other
tools such as UV/Vis spectra, zeta potential, and dynamic
light scattering (DLS), which in turn can be applied as output
signals. Herein we present a resettable and multi-readout
logic system capable of several types of logic operations based
on the aggregation of spiropyran-modified gold nanoparticles
(spiropyran-AuNPs) in aqueous media.
visible light, the spiropyrans exist in their colorless, non-
planar, and closed form (SP), which can be isomerized to the
red, planar, and open merocyanine form (MC) with UV light
À
irradiation by the heterocyclic ring cleavage of the spiro C O
bond.[5] The MC isomer has a phenolate group, which can
readily bind with metal ions. The chelated metal ions can be
released by visible light irradiation.[6] We expect the closed
form of spiropyran-AuNPs to be monodispersed in solutions
that exhibit red color; after UV light irradiation followed with
the addition of 10 mm of Cu2+, the formation of a sandwich
between Cu2+ and MC on different AuNPs can cause open
merocyanine-modified AuNPs (MC-AuNPs) to aggregate,
with a color change from red to purple. We expect that the
combination of UV light irradiation and the subsequent
addition of Cu2+ to spiropyran-AuNPs solutions could be
applied to construct an AND logic gate. Moreover, we can
apply this system to construct other kinds of logic gates based
on various input signals such as different kinds of metal ions
and chelating ligands. The aggregates of MC-AuNPs can be
redispersed upon exposure to visible light irradiation owing to
the open-to-closed isomerization of spiropyran and the
release of the “bridge” (metal ions), which generates a
resettable AuNP-based logic system (Scheme 1).
To realize the spiropyran-AuNP-based logic gates, we
synthesized spiropyran-terminated alkanethiols (Supporting
Information, Scheme S1), and tested the isomerization of
spiropyran thiols in solution. We prepared the solution of the
spiropyran thiols in ethanol at a concentration of 1 ꢀ 10À4 m.
Spiropyran thiols were stored in the dark overnight at 08C,
and the majority of the spiropyran molecules existed as the
closed colorless isomer, which was switched into the open MC
form with UV light irradiation; the maximum absorption at
550 nm was obtained after persistent irradiation for 20
seconds (Supporting Information, Figure S1). Addition of
Cu2+ (1 ꢀ 10À3 m) into the MC solution resulted in a change in
the absorbance spectrum, with a new peak appearing at
420 nm along with the decrease of the absorption at 550 nm,
which is due to the formation of the MC–ion complex
(Supporting Information, Figure S2).[7]
Spiropyrans are an important class of photoswitchable
organic molecules. Under dark conditions or exposure to
[*] D. B. Liu, W. W. Chen, K. Sun, Prof. K. Deng, Prof. W. Zhang,
Prof. Z. Wang, Prof. X. Y. Jiang
CAS Key Lab for Biological Effects of Nanomaterials and
Nanosafety, National Center for NanoScience and Technology
11 Beiyitiao, ZhongGuanCun, Beijing 100190 (China)
Fax: (+86)10-8254-5631
E-mail: wangz@nanoctr.cn
xingyujiang@nanoctr.cn
AuNPs were prepared and modified with an alkanethiol
terminated in triethylene glycol (EG3) and another alkane-
thiol terminated in spiropyran (EG3SP; Scheme 1). EG3
thiols were used to mix with EG3SP thiols to increase the
water solubility of spiropyran-AuNPs and decrease the
density of EG3SP on AuNP surface.[8] The mixed alkanethiols
adsorbed onto gold surfaces by means of ligand exchange.[9]
We found that if the ratio of EG3SP to EG3 was higher than
1:10, the AuNPs can readily aggregate in the process of
preparing spiropyran-AuNPs. Upon decreasing the ratios
from 1:10 to 1:100, the spiropyran-AuNPs can be well-
D. B. Liu, W. W. Chen, K. Sun
Graduate University of Chinese Academy of Sciences Shijingshan
Yuquan Road, 19(A), Beijing 100049 (China)
[**] We thank the Chinese Academy of Sciences, the National Science
Foundation of China (90813032, 20890020, 20933008, 21025520),
the Chinese Academy of Sciences (KJCX2-YWM04, KJCX2-YW-M15),
the Scientific Research Foundation for the Returned Overseas
Chinese Scholars, State Education Ministry, and the Ministry of
Science and Technology (2009CB30001, 2011CB933201) for finan-
cial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201008198.
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Scheme 1. Proposed mechanism for the photoresponsive control of
spiropyran-AuNPs and the molecular structures of stabilizing agents
(EG3) and the closed form of spiropyran ligands (C11SP, EG3SP,
EG6SP) and their corresponding MC isomers (C11mC, EG3MC,
EG6MC). UV light wavelength: 365 nm, visible light: 520 nm. The
concentration of Cu2+ was 10 mm, and the reversible modulations took
place in an aqueous solution (pH 7.0).
Figure 1. The spiropyran-AuNP-based resettable AND logic gate.
a) Design strategy. In the absence of both inputs (0/0) or in the
presence of either input (1/0, 0/1), the spiropyran-AuNPs solutions
remained red (output=0); in the presence of both inputs (1/1), the
color of the spiropyran-AuNP solutions changed from red to purple.
b) Photograph of AuNP solutions and c) their corresponding UV/Vis
absorption. d) A truth table of the AND logic gate.
dispersed in aqueous solution. However, the absorption
response relied on the ratios, with relatively high ratios
(such as those over 1:20), and the spiropyran-AuNPs aggre-
gated quickly and completely after UV light irradiation
followed by the addition of Cu2+ ions. If we decreased the
ratios (such as to 1:60, 1:100), the spiropyran-AuNPs aggre-
gated incompletely; smaller A670/A520 values indicate incom-
plete aggregation (Supporting Information, Figure S3, where
A670 is the shoulder absorption band at 670 nm and A520 is that
at 520 nm). Therefore, we chose 1:10 as the best ratio for
subsequent experiments.
Based on the above results, we first designed an AND
logic gate that employs 30 s of UV light irradiation and the
addition of Cu2+ ions (10 mm) as inputs, and the color change
of solutions containing spiropyran-AuNPs as the output. With
respect to inputs, the presence and absence of UV light
irradiation and the addition of Cu2+ is defined as “1” and “0”,
respectively. For output, we define the red solutions contain-
ing well-dispersed spiropyran-AuNPs as “0”, and the purple
solutions containing aggregates of spiropyran-AuNPs as “1”.
Only the presence of both inputs (1/1) could cause spiro-
pyran-AuNPs to aggregate along with a color change from red
to purple (output = 1); while those in the absence of both
inputs (0/0) or in the presence of either input (1/0, 0/1) remain
red (output = 0; Figure 1b). Furthermore, the AND logic gate
was further confirmed by testing the UV/Vis absorption
responses, a higher A670/A520 value for the presence of both
inputs (1/1) indicated complete aggregation of spiropyran-
AuNPs, while the other three much lower A670/A520 values for
the other kinds of inputs (0/0, 1/0, 0/1, respectively) indicated
the dispersion of spiropyran-AuNPs (Figure 1c,d).
As one of the inputs for this AND logic gate, the choice of
metal ions was critical. We investigated the responses of
spiropyran-AuNPs in the presence of various metal ions,
including Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+,
Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+ (Supporting Informa-
tion, Figure S4) at concentrations of 10 mm, 30 mm, 60 mm,
100 mm, and 300 mm. When the concentrations of metal ions
were higher than 100 mm, almost all kinds of ions can cause the
aggregation of the MC-AuNPs. When the concentrations of
metal ions were less than 100 mm, only Cu2+ caused the
aggregation. This result indicates that the selectivity of MC-
AuNPs towards Cu2+ was concentration-dependent.
To explore the origin of this selectivity toward Cu2+, we
performed theoretical calculations using density functional
theory (DFT) to evaluate the change of the Gibbs free energy
(DG) of the interactions between the MC isomers and various
metal ions. The result showed that DG of Cu2+, Co2+, and Ni2+
were much lower than those of other metal ions (Supporting
Information, Table S1). The interactions between the MC
isomers and Cu2+, Co2+, and Ni2+ tend to take place more
spontaneously than other metal ions. Furthermore, the water-
exchange rate constant of Cu2+ is 4.4 ꢀ 109 sÀ1, which is much
higher than those of Co2+ (3.2 ꢀ 106 sÀ1) and Ni2+ (3.2 ꢀ
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104 sÀ1),[10] indicating that kinetically, Cu2+ has more
chance to bind with MC isomers than other metal ions.
The analysis based on both thermodynamics and kinetics
agrees with our observation that Cu2+ has stronger
ability than other metal ions to bind with the MC isomer
(Supporting Information, Figure S5).
We evaluated the lowest concentration of Cu2+ that
ensured the functions of the AND logic gate. We added
various concentrations of Cu2+ (0 mm, 0.1 mm, 0.3 mm,
0.6 mm, 1.0 mm) into MC-AuNP solutions, and found that
for the color change from red to purple by an increase of
Cu2+, the lowest concentration of Cu2+ that can be
visualized by the naked eye was 0.3 mm (Supporting
Information, Figure S6A), which was demonstrated by
testing their absorption responses (Supporting Informa-
tion, Figure S6B). Furthermore, we found that different
counterions had negligible influences to the chelation
between Cu2+ and MC on AuNPs (Supporting Informa-
tion, Figure S7). Owing to the good selectivity and
sensitivity, this system has the capacity to detect Cu2+ in
aqueous media. We note that the US Environmental
Protection Agency has set the maximum contaminant
level of Cu2+ in drinking water to be 20 mm,[11] a value
that readily allows naked-eye-based readout by this logic
system.
Based on the fact that higher concentrations
(>100 mm) of other metal ions such as Fe3+ can also
induce MC-AuNPs to aggregate, we constructed an OR
logic gate by using MC-AuNPs, 200 mm of Fe3+, and
200 mm of Cu2+. We employed the addition of either Fe3+
or Cu2+ as input and defined the presence and absence of
Fe3+ or Cu2+ as “1” and “0”, respectively. The color
change of MC-AuNP solution was defined as output. In the
presence of either or both inputs (1/0, 0/1, 1/1), the color of
MC-AuNPs solution changed from red to purple immediately
(output = 1) (Figure 2a), which was further demonstrated by
testing their UV/Vis absorption responses, that is, much
higher A670/A520 values for the inputs (1/0, 0/1, 1/1) than that in
the absence of inputs (0/0; Figure 2b,c).
Based on MC-AuNPs, an INHIBIT logic gate can be
created by employing ethylenediaminetetraacetic acid
(EDTA, 100 mm) as one input, the addition of 10 mm of Cu2+
as another input, and the color change of MC-AuNPs solution
was defined as output. The addition of EDTA (input = 1/0)
into solutions containing MC-AuNPs cannot lead to aggre-
gation of MC-AuNPs. However, EDTA has much stronger
capacity than MC isomers to sequester Cu2+, thus preventing
the formation of aggregates in solution containing both MC-
AuNPs and Cu2+ (input = 1/1). Only the addition of Cu2+
(input = 0/1) can cause MC-AuNPs to aggregate (output = 1;
Figure 2d). The INHIBIT logic gate was demonstrated by
testing the UV/Vis absorption responses, a higher A670/A520
value for the presence of Cu2+ (input = 0/1) than those for
inputs (0/0, 1/0, 1,1) (Figure 2e,f).
Figure 2. The MC-AuNP-based resettable OR (a, b, c) and INHIBIT (d, e, f)
logic gates. a) Photograph of MC-AuNP solutions for the OR logic gate in the
absence of inputs (0/0) and in the presence of either or both inputs (1/0,
0/1, 1/1), and b) their corresponding UV/Vis absorption responses. c) A truth
table of the OR logic gate. d) Photograph of MC-AuNP solutions for the
INHIBIT logic gate and e) their corresponding UV/Vis absorption responses.
With respect to the inputs (0/0, 1/0, 1/1), the MC-AuNPs solutions remained
red (output=0); only the presence of Cu2+ (input=0/1) led to the color of
the MC-AuNP solutions changing from red to purple. f) A truth table of the
INHIBIT logic gate.
AuNPs with UV light (30 s); the color of the solution
remained red, indicating that MC-AuNPs still remained
well-dispersed. After incubating with Cu2+ or Fe3+
(>100 mm), the color of the solution changed from red to
purple immediately. To investigate the reversible potential,
aqueous solutions containing aggregates of MC-AuNPs were
centrifugated to remove extra Cu2+ or Fe3+, to which
deionized water was added to disperse the aggregates. To
eliminate the H+-induced aggregation by its interaction with
the ether units of EG3 on AuNPs,[12] we adjusted the pH value
of the new solution to be slightly basic (pH 9.0–10.0), and
irradiated it with visible light (30 s). We found that this
treatment led to a change in the color of the solution from
purple to red within several minutes; this result is expected
because the aggregates were redispersed to be separate
spiropyran-AuNPs after the open-to-closed isomerization of
spiropyran molecules on gold surfaces.
To further confirm these reversible modulations of
spiropyran-AuNP aggregates, we compared the TEM
images of spiropyran-AuNPs before and after UV light
irradiation followed by the addition of Cu2+ (Supporting
Information, Figure S8). The spiropyran-AuNPs can be
photoswitched between dispersed and aggregated forms by
applying UV light irradiation followed with the addition of
Cu2+ and visible light alternatively at least three times. It is at
present difficult to obtain more cycles because of photo-
fatigue of spiropyran on AuNP surfaces.[6c] UV/Vis absorption
Owing to the reversible chemical nature of the spiropyran
group, these AuNP-based logic gates could be reset to their
initial conditions by visible light, under which the aggregation
of spiropyran-AuNPs can be modulated reversibly. We
irradiated the aqueous solution containing spiropyran-
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ions) in basic solution can compete with phenolate
groups on MC to bind with Cu2+. When the pH value
reached 11.0, the binding between Cu2+ and MC on
Au surfaces cannot take place, and AuNPs remained
well-dispersed in aqueous solutions. Thus the optimal
pH range for this logic system was from 4.0 to 8.0
(Supporting Information, Figure S11).
To investigate the influence of the length of the
units of EG that tethered spiropyran and alkanethiol,
we synthesized another two spiropyran ligands con-
taining various lengths of the units of EG (Support-
ing Information, Scheme S2 and S3), one having no
EG (C11SP; Scheme 1), and the other containing six
EGs (EG6SP; Scheme 1). We used EG3 as the
stabilizing agents for all three kinds of spiropyran
Figure 3. a) UV/Vis absorption spectra reflecting the cycles of UV/visible light
irradiation (from a to f, 30 s each). b) The sizes (diameter d) of dispersed (A, C,
E) and aggregated (B, D, F) spiropyran-AuNPs measured by DLS.
spectra confirmed these reversible processes; that is, with the
formation of AuNP aggregates, the absorption band at 520 nm
decreased along with the appearance of a new shoulder
absorption band at about 670 nm (Figure 3a, curves of B, D,
F). With the redispersion of aggregates after visible light
irradiation, the absorption band at 520 nm increased along
with the decrease of those at 670 nm (Figure 3a, curves of A,
C, E). The reversible modulations were also supported by the
DLS data (Figure 3b). The average hydrodynamic diameter
of well-dispersed spiropyran-AuNPs is about 24 nm, while
that of their aggregates increased to be approximately
300 nm, congruent with the TEM analyses.
We also used zeta potential measurements to compare the
surface charge on monodispersed spiropyran-AuNPs and
their aggregates at pH 7.0. The zeta potential of well-
dispersed spiropyran-AuNPs was about À30 mV, while that
of their aggregates was approximately 0 mV (Supporting
Information, Figure S9). We reasoned that the increase of
zeta potentials was due to the positively charged ammonium
group on MC isomers, where the phenolate anionic groups
chelated with Cu2+. These results from zeta potentials are
consistent with those from UV/Vis spectroscopy, TEM, and
DLS data. Importantly, we believe that most of the means for
characterization, such as UV/Vis spectra, zeta potential, and
DLS can serve as output signals in the logic gate, by which a
multi-readout logic system could be achieved.
Several factors influence this logic system. First, pH values
of the solution can greatly influence the stability of spiro-
pyran-AuNPs. We prepared aqueous solutions containing
spiropyran-AuNPs with pH values from 2.0 to 12.0. When the
pH values of the solutions decreased below 4.0, the A670/A520
values was much higher owing to the aggregation of AuNPs.
The ether units of EG3 residing on different AuNPs can
protonate, resulting in aggregation.[12] The spiropyran-AuNPs
were well-dispersed in aqueous solutions with pH values
higher than 4.0 (Supporting Information, Figure S10). Next,
we investigated the influence of pH values to this aggregation
by characterizing the absorbance responses to various pH
values. In the range between pH 4.0 and 8.0, the functional-
ized AuNPs, capped by the mixture of EG3SP and EG3 with a
ratio of 1:10, aggregated in tens of seconds after UV light
irradiation and incubation with Cu2+. We note that aggrega-
tion of spiropyran-AuNPs was incomplete with the increase of
pH values from 8.0, because the negative charges (hydroxide
ligands (the ratio was 1:10 for all the three ligands to EG3).
For the MC-AuNPs having no EG (C11mC-AuNPs), almost
no aggregation was observed upon the addition of Cu2+
(10 mm). We reasoned that the stabilizing agents (EG3) on
AuNP surfaces were too long to allow chelation between Cu2+
and MC on different C11mC-AuNPs surfaces. On the other
hand, when we increased the units of EG to six to form
EG6MC-AuNPs, the aggregation was incomplete in the
present of Cu2+ (10 mm). We reasoned that the EG6 tether
was longer than EG3, thus providing more flexibility for MC
groups on EG6MC-AuNP surfaces; some MC groups on
different EG6MC-AuNPs bind with Cu2+ to enhance the
chances of aggregation, while the other MC groups on the
same EG6MC-AuNPs can also have room to bind with Cu2+
owing to the flexible and long tether, thus resulting in reduced
aggregation. Therefore, EG3SP was the best spiropyran
ligand for this logic system (Supporting Information, Fig-
ure S12).
We finally investigated the effect of irradiation time with
UV light. Photochemistry on a solid surface was different
from that in solution,[13] thus the time required for isomer-
ization for SP to MC on AuNP surface was not consistent with
those in solutions. The spiropyran-AuNPs started to aggre-
gate after 10 s of UV light irradiation, and the aggregation
reached its maximum after a persistent irradiation for about
30 s, plateauing thereafter (Supporting Information, Fig-
ure S13), which is slower than that in solution (20 s; Support-
ing Information, Figure S1). This effect is probably due to the
surface quenching and steric constraints of spiropyran on
AuNP surface that decrease the isomerisation efficiency.[13c]
The optimal irradiation time for spiropyran-AuNPs was thus
about 30 s.
In conclusion, we have presented a spiropyran-AuNP-
based, resettable, multi-readout logic system that includes
AND, OR, and INHIBIT logic operations. The distinctive
advantage of this system is that molecular events in aqueous
solution could be translated into a color change of the
solution, which can be monitored by several readouts, such as
UV/Vis spectroscopy, zeta potential, DLS, and even the
naked eye. Importantly, these logic gates were carried out in
aqueous media, potentiating its applications in biological and
biomedical systems.[14] Moreover, the set–reset function of
these logic gates provides memory elements in an all-aqueous
system, ensuring the entire operation is environmentally
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Received: December 27, 2010
Revised: February 7, 2011
Published online: March 30, 2011
Keywords: copper · gold nanoparticles · logic gates ·
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