Macromolecules 2006, 39, 7461-7463
7461
Signal Amplifying Conjugated Polymer-Based
Solid-State DNA Sensors
Ching-Chin Pun,‡ Kangwon Lee,† Hyong-Jun Kim,† and
Jinsang Kim*,†,‡,§,
Departments of Materials Science and Engineering,
Chemical Engineering, Macromolecular Science and
Engineering, and Biomedical Engineering, UniVersity of
Michigan, Ann Arbor, Michigan 48109
ReceiVed June 14, 2006
ReVised Manuscript ReceiVed September 19, 2006
DNA microarray technology has had a significant impact on
the field of molecular biology and plays an important role in
the diagnosis of diseases,1 drug development,2 and identifying
gene expression.3 Low cost, high sensitivity, high selectivity,
fast detection process are the desired characteristics and are also
the demanding challenges in the development of DNA micro-
arrays. However, the current detection method of the conven-
tional DNA microarrys cannot provide sufficiently sensitive
detection because it relies on the fluorescence emission of the
dye on the analyte DNA. Thus, the sensory signal is simply
proportional to the number of dye-labeled analytes recognized
by the probe DNAs on the microarry.4 Therefore, it is chal-
lenging to detect small amounts of target DNA and target
molecules always need to be duplicated to certain amounts by
polymerase chain reaction (PCR). Furthermore, costly and time-
consuming labeling and subsequent stringent purification of the
target molecules are also required. This makes current DNA
microarray not suitable for the real-time organism detection and
fast diagnosis of gene-related diseases.
Conjugated polymers are emerging materials for biological
sensor applications because of their signal amplification property
and environmental sensitivity.5 Moreover, controlled assembly
of fluorescent sensory polymers expands the dimensionality of
the energy transport properties from 1-D to 2-D and to 3-D
efficiently, augmenting the intrinsic high sensitivity even
further.6 We have been developing self-signal amplifying DNA
microarrays. In this contribution we present the design principle
of conjugated polymer-based signal amplifying DNA sensory
films. We designed and synthesized a conjugated poly(p-
phenyleneethynylene) (PPE) which has carboxylic acid side
chains for bioconjugation with DNAs and alternating hydro-
phobic and hydrophilic side chains for Langmuir-Blodgett (LB)
film fabrication. Scheme 1 illustrates the entire processes
including the LB deposition, bioconjugation with probe DNAs,
and the amplified detection principle.
The PPE for this study was synthesized by the Sonogashira-
Hagihara coupling reaction.7 Scheme 2 shows the polymerization
process and the chemical structure of the PPE. We designed
the PPE to have an amphiphilic property so that it can form a
well-defined thin layer by the LB method. The first repeating
unit has hydrophilic triethylene glycol side chains while the other
repeat unit has hydrophobic side chains of ethyl-protected
carboxylic acid, which are linked to the PPE backbone via the
Figure 1. (a) The pressure-area isotherm and (b) the corresponding
molecular model of the PPE.
hydrophobic hexyl unit. After polymerization the ethyl protec-
tion group was removed to give free carboxylic acid groups
for bioconjugation with amine-modified DNA sequence.
The polymer was dissolved in chloroform at the concentration
of 1 mg/mL. The polymer solution was spread at the air-water
interface of a NIMA 112D trough. We studied the pressure-
area isotherm to understand the conformation and the molecular
packing of the PPE in the Langmuir monolayer. This PPE is
believed to have the face-on structure due to its alternating
hydrophobic and hydrophilic side chains because the sym-
metrically attached hydrophilic and hydrophobic side chains will
position the mainchain phenyl ring co-facial to the air-water
interface.8 Figure 1 shows the pressure-area (π-A) isotherm
and a molecular model of the PPE. The limiting area (A0) of
the π-A isotherm that can be obtained by extrapolating the slope
of the π-A isotherm is about 270 Å2/repeat unit, which is the
surface area the repeat unit of the PPE occupies at the air-
water interface and matches well with the molecular model.8
As the polymer is compressed, the surface pressure increases
until the polymer folds into multilayers at the area per repeat
units of about 50 Å2. Because of the surfactant design, the π-A
isotherm is completely reversible during the compression and
expansion cycles even after the monolayer folds into multilayers.
† Department of Materials Science and Engineering.
‡ Department of Chemical Engineering.
§ Department of Macromolecular Science and Engineering.
Department of Biomedical Engineering.
The Langmuir monolayer of the PPE was transferred to a
1,1,1,3,3,3-hexamethyldisilazane (HMDS)-treated hydrophobic
* Corresponding author: Tel 734-936-4681; Fax 734-763-4788; e-mail
10.1021/ma061330s CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/04/2006