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Selective Colorimetric Detection of
Polynucleotides Based on the Distance-Dependent
Optical Properties of Gold Nanoparticles
Robert Elghanian, James J. Storhoff, Robert C. Mucic,
Robert L. Letsinger,* Chad A. Mirkin*
A highly selective, colorimetric polynucleotide detection method based on mercaptoal-
kyloligonucleotide-modified gold nanoparticle probes is reported. Introduction of a sin-
gle-stranded target oligonucleotide (30 bases) into a solution containing the appropriate
probes resulted in the formation of a polymeric network of nanoparticles with a con-
comitant red-to-pinkish/purple color change. Hybridization was facilitated by freezing
and thawing of the solutions, and the denaturation of these hybrid materials showed
transition temperatures over a narrow range that allowed differentiation of a variety of
imperfect targets. Transfer of the hybridization mixture to a reverse-phase silica plate
resulted in a blue color upon drying that could be detected visually. The unoptimized
system can detect about 10 femtomoles of an oligonucleotide.
Sequence-specific methods for detecting
polynucleotides are critical to the diagnosis
of genetic and pathogenic diseases (1).
Most detection systems make use of the
hybridization of an immobilized target
polynucleotide with oligo- or polynucleo-
tide probes containing covalently linked
reporter groups (2–5). Radioactive
32
Por
35
S labels in polynucleotide detection offer
exquisite sensitivity and are commonly used
to follow hybridization (3). However, radio-
active probes create disposal problems, re-
quire specially trained personnel, and have
a short shelf life. Increasingly, radioactive
atoms are being replaced by nonradioactive
organic reporter groups, which are detected
by their color, fluorescence, or lumines-
cence (2, 3, 5). The organic groups may
either be covalently attached to the probes
or may be generated in solution by enzymes
that are bound to the probe. Each of these
strategies has advantages and disadvantages,
depending on the target DNA molecules
and detection environments, and no single
method has gained supremacy (2).
Herein, we report a highly selective col-
orimetric detection technology for poly-
nucleotides that differs fundamentally from
previously described sensor systems in sev-
eral respects. First, mercaptoalkyloligo-
nucleotide-modified Au nanoparticles are
used as reporter groups rather than radioac-
tive atoms or organic substituents. Second,
hybridization results not only in the binding
of an oligonucleotide probe to the target
sequence, but also in the formation of an
extended polymeric network in which the
reporter units are interlocked by multiple,
short duplex segments (Fig. 1). Third, the
signal for hybridization is governed by the
optical properties of the nanoparticles,
which depend in part on their spacing with-
in the polymeric aggregate. Nanoparticle
aggregates with interparticle distances sub-
stantially greater than the average particle
diameter appear red, but as the interparticle
distances in these aggregates decrease to less
than approximately the average particle di-
ameter, the color becomes blue (6). This
shift, attributed to the surface plasmon res-
onance of the Au, has been observed in
other non–oligonucleotide-based strategies
for organizing nanoparticles into aggregate
structures (7) and has been studied theoret-
ically (8). Gold particles ;13 nm in diam-
eter (9) were chosen because they can be
readily prepared with little deviation in size
(62 nm) and exhibit a sharp plasmon ab-
sorption band (maximum absorbance at
wavelength 520 nm). Two additional fac-
tors proved helpful in developing this tech-
nology: (i) hybridization is greatly acceler-
ated by heating or freezing the solutions and
(ii) the color changes associated with hy-
bridization are significantly enhanced and
easily visualized if the hybridized sample is
“developed” on a solid support. With these
techniques, target polynucleotides can be
identified rapidly without instrumentation.
The use of mercaptoalkyloligonucle-
otide-modified Au nanoparticles in any
sensing scheme requires that they exhibit
long-term stability in the presence of high
molecular weight DNA and in solutions
that span a wide range of electrolyte con-
centration (0.1 to 1 M NaCl). The nano-
particles previously used in our material
syntheses strategies (10) do not exhibit this
type of long-term environmental stability.
However, with the current preparation pro-
cedure, one can synthesize Au nanopar-
ticles modified with oligonucleotide of vari-
able length and sequence that exhibit long-
term stability in 1 M NaCl solutions and in
solutions containing macromolecular salm-
on sperm DNA (up to 100 mg of salmon
sperm DNA can be added to a solution
containing 50 ml of each probe prepared
without interfering with the sensor applica-
tions). In our protocol, nanoparticles are
loaded with 59- or 39-mercaptoalkyloligo-
nucleotides in solution without added salt,
aged in 0.1 M NaCl for 40 hours in the
presence of the mercaptoalkyloligonucleo-
tides to increase the loading, collected by
centrifugation, and resuspended. This pro-
cedure was used to prepare all of the nano-
particle probes described herein.
Initial studies were carried out with a
three-component system, wherein two
probes, 1 and 2, were used for one target
sequence, 3 (Fig. 2A). Each nanoparticle
has many molecules of a 28-base oligomer
linked by a thiol tether at the 59 terminus to
its surface (11). The first 13 nucleotides
(not shown) of the 28-base oligomers serve
as a flexible spacer (12), and the last 15
serve as a recognition element for the tar-
get. Sequences were selected such that on
hybridization, the recognition segments of
probes 1 and 2 could align contiguously on
the target 3.
For this system, hybridization was slow
when the components (probes and target)
(13) stood in 0.1 M NaCl at room temper-
ature, but after standing overnight, the so-
lution changed from red to reddish purple.
The slow rate of the reaction may be due to
steric considerations and the high negative
charge density created by the extensive oli-
gonucleotide loading on the nanoparticle
surfaces. A similar color change could be
Department of Chemistry, Northwestern University,
Evanston, IL 60208, USA.
*To whom correspondence should be addressed. E-mail:
camirkin@chem.nwu.edu and r-letsinger@chem.nwu.edu
Fig. 1. Schematic representation of the concept
for generating aggregates signaling hybridization
of nanoparticle-oligonucleotide conjugates with
oligonucleotide target molecules. The nanopar-
ticles and the oligonucleotide interconnects are
not drawn to scale, and the number of oligomers
per particle is believed to be much larger than
depicted.
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effected rapidly, however, by warming the
mixture at 50°C for 5 min or freezing it in a
bath of dry ice and isopropyl alcohol and
then thawing it at room temperature. The
acceleration in rate caused by freezing likely
reflects the development of high local con-
centrations of salt, the oligonucleotide tar-
get, and nanoparticles within pockets in the
ice structure (14). Control experiments
showed that the transition requires the
presence of all three components (both
types of nanoparticle conjugates and the
target oligonucleotide), and in absence of
hybridization, the probes are not affected by
the process, as evidenced by ultraviolet-
visible spectroscopy and transmission elec-
tron microscopy.
A “melting curve” obtained at a wave-
length of 260 nm on a nanoparticle-DNA
aggregate hybridized by the freeze-thaw cy-
cle gave a “melting” temperature (T
m
)of
57°C (Fig. 3A, black circles), as compared
with T
m
5 56°C for a solution of free
oligonucleotides hybridized in solution at
room temperature in the absence of nano-
particles (Fig. 3A, red squares). The curve
for the Au-nanoparticle/DNA complex is
remarkably steep; the temperature range for
melting [full width at half maximum
(FWHM) 5 4°C] is narrow compared with
the temperature range for dissociation of
the complex formed from the free oligonu-
cleotides (FWHM 5 12°C) [Fig. 3A, insets
(15)]. Corresponding sharp transitions at
the same temperature but with a drop in
absorbance rather than an increase were
obtained when the dissociation of the
nanoparticle aggregates was observed at 620
and 700 nm. All of the experiments with
the nanoparticle conjugates measure chang-
es in the optical properties of the nanopar-
ticles (not the DNA). The oligonucleotides
do not absorb at 620 and 700 nm, and the
concentration of the target oligonucleotide
in the colloid solution is too low to account
for the absorbance change at 260 nm.
We attribute the spectral changes in this
nanoparticle system to the reversible forma-
tion and dissociation of aggregates formed
by hybridization of the covalently attached
probe segments with the target oligonucle-
otide. Hybridization results in decreased in-
terparticle distances with a concomitant
change in color and formation of extended
polymeric nanoparticle aggregates (16). On
warming, some of the DNA cross-links can
dissociate without dispersing the Au nano-
particles into solution. Because the signal
depends on the nanoparticle spacing, our
melting analysis is insensitive to these ini-
tial DNA dissociation events, and the ob-
served temperature range for “melting” is
unusually narrow. The size of the aggregates
in these systems may also influence the
colorimetric change, but this issue has not
yet been addressed.
A simpler way to monitor hybridization
is to spot 1 to 3 ml of the solution con-
taining the Au-nanoparticle/DNA aggre-
gates on a C
18
thin-layer chromatography
(TLC) plate (Fig. 3B). Initially, the spot
retains the color of the solution mixture,
which ranges from red in the absence of
hybridization through reddish-purple to
purple on hybridization, depending on the
system. On drying, a uniform blue spot
develops if the Au-oligonucleotide conju-
gates have been linked by hybridization
with the target oligonucleotide; in the
absence of an appropriate target or with
solutions spotted above thermal denatur-
ation conditions, the spot remains pink.
The color differentiation is enhanced by
the C
18
-silica support, a phenomenon at-
Fig. 2. Mercaptoalkyloligonucleotide-modified
13-nm Au particles and polynucleotide targets
used for examining the selectivity of the nanopar-
ticle-based colorimetric polynucleotide detection
system. (A) Complementary target; (B) probes
without the target; (C) a half-complementary tar-
get; (D) a 6-bp deletion; (E) a 1-bp mismatch; and
(F) a 2-bp mismatch. For the sake of clarity, only
two particles are shown; in reality a polymeric ag-
gregate with many particles is formed. Dashed
lines represent flexible spacer portions of the mer-
captoalkyloligonucleotide strands bound to the
nanoparticles; note that these spacers, because
of their noncomplementary nature, do not partic-
ipate in hybridization. The full sequences for the
two probes, 1 and 2, which bind to targets 3
through 7, are
1-59SH-~CH
2
!
6
- @CTA-ATC-CGC-ACA-G#
spacer
@CC-TAT-CGA-CCA-TGC-T#
probe
2-59SH-~CH
2
!
6
-@ATG-GCA-ACT-ATA-C#
spacer
@GC-GCT-AGA-GTC-GTT-T#
probe
58.0°C
59.0°C
58.5°C
20 40 60 20 40 60
10 20 30 40
Temperature (°C)
50 60 70
T
m
= 57°C
T
m
= 56°C
1.00
0.95
0.90
0.85
Absorbance at 260 nm (normalized)
AB
Fig. 3. (A) Comparison of
the thermal dissociation
curves for complexes of
mercaptoalkyloligonucleo-
tide-modified Au nanopar-
ticles (black circles) and
mercaptoalkyloligonucleo-
tides without Au nanopar-
ticles (red squares) with the
complementary target, 3,in
hybridization buffer (0.1 M
NaCl, 10 mM phosphate
buffer, pH 7.0). For the first
set (black circles), a mixture
of 150 ml of each colloid
conjugate and 3 mlofthe
target oligonucleotide in hy-
bridization buffer (0.1 M
NaCl, 10 mM phosphate, pH 7.0) was frozen at the temperature of dry ice, kept for 5 min, thawed over
a period of 15 min, and diluted to 1.0 ml with buffer (final target concentration, 0.02 mM). The
absorbance was measured at 1-min intervals with a temperature increase of 1°C per minute.
The increase in absorption at 260 nm (A
260
) was ;0.3 absorption units (AU). In the absence of the
oligonucleotide targets, the absorbance of the nanoparticles did not increase with increasing
temperature. For the second set, the mercaptoalkyloligonucleotides and complementary target
(each 0.33 mM) were equilibrated at room temperature in 1 ml of buffer, and the changes in
absorbance with temperature were monitored as before. The increase in A
260
was 0.08 AU. (Insets)
Derivative curves for each set (15). (B) Spot test showing T
c
(thermal transition associated with the
color change) for the Au nanoparticle probes hybridized with complementary target. A solution
prepared from 150 ml of each probe and 3 ml of the target (0.06 mM final target concentration) was
frozen for 5 min, allowed to thaw for 10 min, transferred to a 1-ml cuvette, and warmed at 58°C for
5 min in the thermally regulated cuvette chamber of the spectrophotometer. Samples (3 ml) were
transferred to a C
18
reverse phase plate with an Eppendorf pipette as the temperature of the solution
was increased incrementally 0.5°C at 5-min intervals.
REPORTS
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tributable to increased aggregation of the
preorganized polynucleotide-linked nano-
particles upon drying the solution on the
support. Moreover, the solid support pre-
vents samples heated above the DNA dis-
sociation temperature from rehybridizing;
therefore, the colors are indefinitely sta-
ble, and the plates provide a permanent
record of the test.
A striking feature of this test is the
sharpness and clarity of the transition
(Fig. 3B). The spot was blue for a hybrid-
ized sample containing 1, 2, and 3 heated
in solution to 58°C before the test, indi-
cating that the nanoparticles were still
assembled at this temperature. However,
the spot was pink when the sample was
heated to 59°C, a temperature at which
the assembly is unstable and the nanopar-
ticles are dispersed in solution. At 58.5°C,
the color was intermediate. We term this
the colorimetric temperature T
c
, which is
close to but slightly higher than T
m
(57°C) for the polymeric aggregate, as de-
termined by the change in the ultraviolet
spectrum for the solution (Fig. 3A). Rep-
etitions of the spot test with another set of
probes prepared independently afforded
the same narrow range for the color
change with T
c
5 58.0°C.
The small temperature range for the
change in color enables one to discriminate
between the perfect complement (3) and
the mismatched targets (4 to 7) (Figs. 2 and
4). In each test, the probe-target mixtures
were frozen, thawed at room temperature,
warmed in a water bath at the indicated
temperature for 5 min, and then spotted on
aC
18
-silica plate. At 53°C, the test was
positive (blue) for the fully matched target
(Figs. 2A and 4A) but negative (pink) for
the target with one mismatched base (Figs.
2E and 4E). At 42°C, it was positive for the
target with one mismatched base (Figs. 2E
and 4E) but negative for targets with two
mismatches (Figs. 2F and 4F) or a six-base
deletion (Figs. 2D and 4D). As expected,
tests with the target absent (Figs. 2B and
4B) or with extensive mismatches in the
segment targeted by one of the probes (Figs.
2C and 4C) were negative.
This system of nanoparticles, aggre-
gates, and oligonucleotides is complex. At
this stage we treat these colorimetric tran-
sition temperatures as empirical values
useful in detecting and differentiating
oligonucleotides with specific sequences.
To examine the dependence of the results
on the hybridization procedure, we, rather
than freezing the solutions, warmed indi-
vidual samples of targets 3 through 7 with
solutions containing 1 and 2 to 65°C (a
temperature above the dissociation tem-
peratures of the complex formed with the
fully matched target) and allowed the so-
lutions to cool slowly with intermittent
pauses of 5 min at 62°, 53°, 42°, 38°, and
25°C. At the end of each pause, the sam-
ples were removed and spotted as before.
The plate obtained by this procedure was
identical to that in Fig. 4, except that 7
gave a blue rather than pink spot at 38°C
(when spotted at 39.5°C, however, the
color was pink, indicative of dissociation).
This experiment shows that there is con-
siderable latitude in conditions for carry-
ing out the assays, although the absolute
T
c
values may vary slightly with changes
in the method for hybridization.
The high degree of discrimination in
these systems may be attributed to two fea-
tures. (i) Alignment of two relatively short
(15 nucleotide) probe oligonucleotides on
the target is required for a positive signal
(Figs. 2C and 4C). A mismatch in either
segment is more destabilizing than a mis-
match for a single, longer probe (such as a
30-nucleotide segment) with the same tar-
get. (ii) As in the spectrophotometric anal-
yses, the colorimetric signal depends on the
formation or dissociation of a polymeric
network of Au nanoparticles held together
by multiple structurally similar tethers (that
is, the oligonucleotide duplexes), which re-
sults in the narrowing of the temperature
range observed for the transition. In princi-
ple, this three-component nanoparticle-
based strategy should be more selective
than any two-component detection system
based on a single-strand probe hybridizing
with the target. Strategies such as ours that
exploit reversible assembly of particles by
hybridization and use probes with signals
sensitive to particle aggregation or distance
should prove generally useful in designing
highly discriminating nucleic acid detec-
tion systems. We do not yet know the ulti-
mate sensitivity of the system, although
with the unoptimized system, ;10 fmol of
an oligonucleotide can be detected (17).
The method should be particularly useful in
assays where expense and simplicity in in-
strumentation and operation are important.
REFERENCES AND NOTES
___________________________
1. S. Razin, Mol. Cell. Probes 8, 497 (1994); J. G. Hacia
et al., Nature Genet. 14, 441 (1996).
2. E. S. Mansfield et al., Mol. Cell. Probes 9, 145 (1995);
L. J. Kricka, Ed., Nonisotopic DNA Probe Tech-
niques (Academic Press, San Diego, 1992).
3. B. D. Hames and S. J. Higgins, Eds., Gene Probes 1
(IRL Press, New York, 1995).
4. J. Wang et al., J. Am. Chem. Soc. 118, 7667 (1996).
5. S. Tyagi and F. R. Kramer, Nature Biotechnol. 14,
303 (1996).
6. U. Kreibig and L. Genzel, Surf. Sci. 156, 678 (1985);
B. Dusemund et al., Z. Phys. D 20, 305 (1991).
7. M. Brust et al., Adv. Mater. 7, 795 (1995); K. C.
Grabar et al., J. Am. Chem. Soc. 118, 1148 (1996);
J. J. Storhoff, R. C. Mucic, C. A. Mirkin, J. Cluster
Sci., 8, 179 (1997).
8. W.-H. Yang, G. C. Schatz, R. P. Van Duyne, J.
Chem. Phys. 103, 869 (1995).
9. K. C. Grabar et al., Anal. Chem. 67, 735 (1995); G.
Frens, Nature Phys. Sci. 241, 20 (1973).
10. C. A. Mirkin et al., Nature 382, 607 (1996).
11. Multiple loading of the oligonucleotides is necessary
for subsequent cross-linking in our system. The sur-
face chemistry involved in linking alkylthiols to Au
surfaces is still a subject of debate [see C. S. Weis-
becker et al., Langmuir 12, 3763 (1996)]. An alterna-
tive method of binding oligonucleotides covalently to
Au particles, which was developed for a different
purpose, was reported to give ;1-nm nanoparticles
containing a single oligomer on a particle [A. P. Alivi-
satos et al., Nature 382 609 (1996)].
12. Subsequent experiments have shown that the oligo-
nucleotide spacer is not essential.
13. The concentrations for the three components were
the same as those described in the caption for Fig.
3A.
14. For another example in which the rate of a reaction
dependent on the hybridization of oligonucleotides
was accelerated by the freezing of an aqueous solu-
tion of the components, see S. M. Gryaznov and
R. L. Letsinger, J. Am. Chem. Soc. 115, 3808 (1993).
15. We calculated the temperature ranges for both melt-
ing analyses depicted in Fig. 3A by measuring the full
width at half maximum for each derivative curve (Fig.
3A, insets). The value of T
m
for the nanoparticle sys-
tem hybridized by the freeze-thaw method agreed
(60.2°C) with that for the nanoparticle system hy-
bridized at room temperature (24 hours).
16. Transmission electron microscopy pictures of typical
aggregates are available in supplementary material.
Similar images can be found in (10).
17. For this experiment, 1 ml of solution containing 10
fmol of target oligonucleotide and 1 ml of solution
containing both nanoparticle probes in a buffer (0.3
A
B
C
D
E
F
25 38 42
Temperature (°C)
53 60
Fig. 4. Selective polynucleotide detection for the
target probes shown in Fig. 2: (A) complementary
target; (B) no target; (C) complementary to one
probe; (D) a 6-bp deletion; (E) a 1-bp mismatch;
and (F) a 2-bp mismatch. Nanoparticle aggregates
were prepared in a 600-ml thin-walled Eppendorf
tube by addition of 1 mlofa6.6mM oligonucleotide
target to a mixture containing 50 ml of each probe
(0.06 mM final target concentration). The mixture
was frozen (5 min) in a bath of dry ice and isopropyl
alcohol and allowed to warm to room temperature.
Samples were then transferred to a temperature-
controlled water bath, and 3-ml aliquots were re-
moved at the indicated temperatures and spotted
onaC
18
reverse phase plate.
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M NaCl, 10 mM phosphate, pH 7.0) were mixed,
frozen (40 s), thawed (5 min), and spotted with a
capillary tube on a C
18
TLC plate. A blue spot devel-
oped. In the absence of target, the color remained
pink. For comparison, the lower limit for detecting an
oligonucleotide with the use of a probe labeled with
fluorescein in a sandwich hybridization system was
reported to be 500 fmol [M. S. Urdea et al., Nucleic
Acids Res. 16, 4937 (1988)].
18. We acknowledge support by grants from the Nation-
al Institute of General Medical Sciences (GM 10265)
and the Office of Naval Research (N0014-94-1-0703
and N00014-97-10430) and the Department of De-
fense (MURI DAAG 55-97-1-0133).
16 April 1997; accepted 24 June 1997
Superconductivity up to 126 Kelvin in Interstitially
Doped Ba
2
Ca
n
–1
Cu
n
O
x
[02(
n
–1)
n
–Ba]
C. W. Chu,* Y. Y. Xue, Z. L. Du, Y. Y. Sun, L. Gao,
N. L. Wu,† Y. Cao, I. Rusakova, K. Ross
A new high-temperature superconducting compound system, Ba
2
Ca
n–1
Cu
n
O
x
[02(n–1)
n–Ba] interstitially doped with calcium or (Ca,Cu) has been identified to exhibit a transition
temperature up to 126 kelvin, the highest for a superconductor without a volatile toxic
element. 02(n–1)n–Ba has body-centered tetragonal symmetry and an unusual charge-
reservoir block. The compounds offer interesting opportunities for high-temperature
superconductivity science and technology.
In the past 10 years, many high-temperature
layered cuprate superconductors have been
discovered (1). They can be represented by
the generic formula A
m
E
2
R
n–1
Cu
n
O
2n1m12
with a stacking sequence of m layers of
(AO) inserted between two layers of (EO)
on top of n layers of (CuO
2
) interleaved by
(n–1) layers of (R), where A, E, and R are
various cations. The generic formula may be
abbreviated in terms of the four-digit classi-
fication scheme (2) m2(n–1)n or A-m2(n–
1)n–E for further subclassification. Present-
ly, all superconductors that have transition
temperatures (T
c
’s) above the liquid nitro-
gen boiling point of 77 K belong to or are
derivable from the following compound
families: (i) Cu-1212-Ba with R 5 Yora
rare-earth element, except Ce, Pr and Tb
(such as CuBa
2
YCu
2
O
7
5 YBa
2
Cu
3
O
7
,
commonly known as Y-123); (ii) A-22(n–
1)n–E with A 5 Bi, Tl, Pb, or Hg, R 5 Ca,
and with an appropriate E 5 Sr or Ba (such
as Bi
2
Sr
2
Ca
2
Cu
3
O
10
); and (iii) A–12(n–
1)n–E with A 5 Tl, Hg, Au, (Cu,C), B, Al,
or Ga, R 5 Ca, and with an appropriate E 5
Ba or Sr (such as TlBa
2
Ca
2
Cu
3
O
9
). Stud-
ies of these compounds reveal that layers
in these compounds can be grouped into
two (3), namely the charge-reservoir block
of (EO)(AO)zzz(AO)(EO), and the active
block of (CuO
2
)(R)(CuO
2
)zzz(R)(CuO
2
). The
charge-reservoir block provides the sources
of charge-carriers for the active block which
is considered the main component for
the superconductivity in the compound.
New high-temperature superconductor fam-
ilies would help further unravel the roles
of the charge-reservoir block, the active
block, or both.
The record T
c
of high-temperature su-
perconducting compounds is 134 K at am-
bient pressure (4)or164Kat30GPa(5).
Unfortunately, those with a T
c
above 120
K contain volatile toxic elements such as
Tl in Tl-2223-Ba, which superconducts at
125 K, and Hg in Hg-1223-Ba, which
becomes superconducting at 134 K. This
poses serious challenges to practical appli-
cations. Consequently (6), Cu-1212-Ba
(or Y-123) remains the most viable mate-
rial for high-temperature superconducting
thin-film devices, in spite of its lower T
c
of
93 K; and Bi-2212-Sr and Bi-2223-Sr for
high-temperature superconducting con-
ductors, operated at temperatures prefera-
bly below 77 K because of their weak flux
pinning force. We report a new supercon-
ducting system, the interstitially doped
Ba
2
Ca
n–1
Cu
n
O
x
[02(n–1)n–Ba], which is
synthesized under pressure and displays a
T
c
of 126 K for n 5 3or117Kforn54.
The 02(n–1)n–Ba compound system with-
out C-inclusion exhibits a body-centered
tetragonal I4 symmetry and a rather open
charge-reservoir block, which offers new
flexibility in modifying the compound. For
instance, intercalation of carbonate, hy-
droxyl, or both ions into the charge-reser-
voir block transforms the 126 K phase into
a more stable 90 K phase.
We searched for high-T
c
compounds
without volatile toxic elements by mod-
ifying the charge-reservoir block. Our
examination of the existing T
c
data re-
veals a simple empirical rule (7) that
compounds with simpler charge-reservoir
blocks, compounds whose charge-reservoir
block contains Ba instead of Sr, and com-
pounds without rare-earth elements, usually
have a higher T
c
. For instance, YBa
2
Cu
3
O
7
displays a T
c
; 93 K, in contrast to
YBa
2
Cu
4
O
8
, which has a more complex
charge-reservoir block and shows a lower T
c
; 80 K and to YSr
2
Cu
3
O
7
, which has two
(SrO) layers in its charge-reservoir block
and shows a lower T
c
, 60 K. A simpler
charge reservoir may be an easier way to
retain the integrity of and to improve the
coupling between the CuO
2
layers in the
active block. The higher T
c
for compounds
with double BaO layers than with double
SrO layers may be associated with the high-
er polarizability of Ba than Sr or the less
possible mixing of Ba in the charge-reser-
voir block with Ca in the active block. The
Ba-based perovskite ferroelectrics also often
have a higher Curie point, below which
polarization appears, than the Sr-based
ones. Therefore, a desirable candidate for a
high T
c
without any volatile toxic element
appears to be the Ba-based compound sys-
tem Ba
2
Ca
n–1
Cu
n
O
x
[02(n–1)n–Ba]. This
compound system in its ideal form consists
of a simple [(BaO)(BaO)] charge reservoir
without any (AO) layers and an active
block of n (CuO
2
) layers separated by (n–1)
Ca layers and is expected to display a body-
centered tetragonal I4 symmetry. Com-
pounds with similar structure were observed
previously in the nonsuperconducting
(PbBa)(YSr)Cu
3
O
8
[or 0223-(PbBa) with
R 5 (YSr)] (8) and the superconducting
system Sr
2
Ca
n–1
Cu
n
O
2n12
[02(n–1)n–Sr]
with a T
c
up to ; 85K(9). The Ba-Ca-
Cu-O system has also been studied previ-
ously by several groups (10–17) and was
found to form the C-stabilized (Cu
1–y
C
y
)Ba
2
Ca
n–1
Cu
n
O
x
[(Cu,C)–12(n–1)n–Ba]
C. W. Chu, Y. Y. Xue, L. Gao, Y. Cao, Texas Center for
Superconductivity and Department of Physics, University
of Houston, Houston, TX 77204 –5932, USA.
Z. L. Du, Y. Y. Sun, N. L. Wu, I. Rusakova, K. Ross, Texas
Center for Superconductivity, University of Houston,
Houston, TX 77204 –5932, USA.
*To whom correspondence should be addressed. E-mail:
cwchu@uh.edu
†On leave from National Taiwan University.
Fig. 1. Field-cooled x(T) for A samples (0223-Ba)
and a B sample (0234-Ba) as synthesized (A and
B) and after exposure to humid air (A9). Inset: r(T);
emu indicates electromagnetic units.
REPORTS
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