Transcript GaAs-molecular interface for quantum semiconductor biosensors

GaAs-Molecular Interface for Quantum Semiconductor Biosensors
G. MARSHALL1, 2, O. Voznyy 1, X. DING 1, D. LEPAGE 1, J.J. DUBOWSKI 1, 3
1Département
de Génie Électrique et de Génie Informatique, Université de Sherbrooke, Sherbrooke, Québec, 2
Institute for Chemical Process and Environmental Technology, National Research Council of Canada, Ottawa,
Ontario 3 Canada Research Chair in Quantum Semiconductors
Motivation: Need to develop optical biosensor for rapid and simultaneous detection (< 15 min) of different pathogenic substances at the point of care
INTRODUCTION
FACTORS AFFECTING THIOL DEPOSITION ON GaAs(001)
Nanocrystals and various nanoparticles interfaced with biological materials are thought to have
potential as novel luminescent probes for both diagnostics (e.g., imaging) and therapeutic (e.g., drug
delivery) applications because of their comparable size to biomolecules and attractive optical, electrical
and magnetic properties. Semiconductor quantum dots (QDs) are of particular interest due to their
unique optical properties, including practically unbleachable fluorescence and wide spectrum coverage
(from 400 nm to 2 μm).
Instead of colloid quantum dots, we employ epitaxial grown QDs (eQDs) array grown directly on
different substrates by thin film deposition technology. eQDs arrays allow for implementation of various
processing techniques that otherwise would be impractical for colloidal QDs. Our device will have a far
greater number of simultaneously resolvable QDs emitting distinct spectra specially designed for ultrasensitive detection of minuscule quantities of different biomolecules. By binding commercially-available
monoclonal antibodies to separate dots in the multicolour array of QDs, we expect to achieve high
specificity and simultaneous detection of different viruses or viral antigens.
Proposed Architecture of Quantum Dots Based Biosensors
GaAs capping layer
Epitaxially grown
quantum dots
(eQDs), e.g., InAs
Our first-principles calculations have shown that chemisorption of thiols happens
via cleavage of S-H bond in a physisorbed precursor state.
 Depending on surface composition this cleavage may be energetically favourable
or not. The amount of sites at which it is favourable determines the speed of SAM
deposition.
 Cleaved hydrogen in most cases stays on surface, or can desorb as molecular H2,
as on gold.
Available experimental results fall into 2 categories
of SAMs: with 57 tilt and 14 tilt.
Our calculations show that 50% thiol – 50% H
adsorption results in tilt of 57 ,
while 100% coverage by thiols results in 14  tilt.
Virus
Antibody
Adsorption mechanism of thiols on GaAs is not well studied and understood.
From available experimental data following 3 main factors affecting deposition can
be highlighted:
 Absence of oxides
 Surface stoicheometry
 pH of the solution
 Functionalization of GaAs surface is of

paramount importance for the
Substrate
application of QDs for biodetection
SURFACE PASSIVATION OF GaAs WITH CH3(CH2)15SH (T16)
Surface Etching and Thiolation Recipes
Thus, the quality of SAM is determined by the success of the preparation technique
to remove H from surface (or to avoid its deposition).
Recipe I
Recipe II
Surface Etching
37% HCl, 1min
NH3.H2O/H2O; HCl/ethanol, no ambient exposure
Thiol Deposition
55oC, with 5% NH3.H2O, N2
Room temperature, no NH3.H2O
BIOTIN FUNCTIONALIZATION OF THE GaAs INTERFACE
The molecular interface with GaAs provides a physical link to the biosensing
elements in addition to stabilization of the surface properties. The proposed
architecture is depicted below (left) consisting of a monolayer of ATA (amineterminated alkanethiol - HS(CH2)11NH2) functionalized with biotin.
XPS Analysis of T16 Passivated GaAs
Recipe I
GaAs-S(CH2)15CH3
N2 atmosphere
55oC
Recipe II
GaAs-S(CH2)15CH3
r.t.
AsOx
48
46
AsOx
44
42
40
38
48
46
Binding Energy (eV)
44
42
40
38
Binding Energy (eV)
 Less oxides were observed in GaAs passivated with T16 using recipe II
Photoluminescence (PL) Decay of T16 Passivated GaAs
200
Recipe I
GaAs
o
GaAs-T16 (55 C)
150
100
50
0
PL Intensity (a.u)
PL Intensity (a.u.)
200
XPS Analysis of the GaAs-ATA-Biotin Architecture
100
50
0
0
400
800
1200
Atmospheric Exposure Time (hours)
I  I 0  Ae
PL Decay Curves are fitted with
Recipe I
 FTIR spectra is consistent with molecular ordering of ATA (above right)
GaAs
GaAs-T16 (r.t.)
150
0
400
800
1200
Atmospheric ExposureTime (hours)
Samples
Recipe II
 t / 1
Recipe II
GaAs
GaAs-T
GaAs
GaAs-T
I0
13.02
34.49
15.96
43.56
A
33.98
85.06
61.48
120.95
1(hours)
293.55
69.26
226.47
587.37
I0+A
47
119.55
77.44
164.51
 GaAs passivated with T16 using recipe II shows higher PL signal and
slower PL decay dynamics, therefore more stable interface.
 Chemical shift of the S 2p doublet (left) is -1. 3 eV relative to the free
state thiol (163.7 eV) indicating charge transfer to the sulphur and the
formation of a covalent bond with the GaAs substrate
 Increase of the N1s signal (right) is characteristic of biotin functionalization
of the amine headgroups. Coverage of the amine is 43%, though steric
repulsion of biotin excludes up to 50% of the available sites
Transmission FTIR Analysis of GaAs-S(CH2)15CH3
CONCLUSIONS
• Effect of deposistion conditions on quality of SAMs is studied theoretically
•Tested recipes show that recipe without ammonia in solution provides better ordered and
more stable in time SAMs.
 Recipe II shows stronger absorption, a narrower peak and lower
energy of the vibrational mode corresponding to a higher degree
of self-assembly of an ordered monolayer
• Same recipe provides similar results for NH2 terminated thiols, avoiding interaction of
NH2 group with surface. These SAMS are used for the following biotinilation of the sample.
•XPS results confirm high amount of biotin attached to the surface, appropriate for further
attachment of avidin.
RQMP annual meeting, Montreal, May 14, 2007