1172
Chemistry Letters Vol.35, No.10 (2006)
Grafting Organic and Biomolecules on H-Terminated Porous Silicon from a Diazirine
Shuai Wei, Jing Wang, Dong-Jie Guo, Ya-Qing Chen, and Shou-Jun Xiaoꢀ
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, P. R. China
(Received July 26, 2006; CL-060845; E-mail: sjxiao@nju.edu.cn)
A diazirine compound, 1,4-(1-azi-2,2,2-trifluoroethyl)ben-
resulted carboxyl group (b) was further converted to an amino-
reactive linker, NHS (N-succinimidyl) ester (c), under the pres-
ence of NHS and an activator DCC (dicyclohexylcarbodiimide)
in 1,4-dioxane, and finally to a protein monolayer (d) of BSA in a
slightly alkaline PBS (phosphate buffered saline) solution
(pH = 9.0). In the final step, the side-chain amino groups of
lysine residues or N-terminals on the protein surface displace
NHS groups, resulting in covalent attachment of the protein.
The slightly alkaline solution was used to deprotonate amines
to enable an easy reaction with the NHS ester.
zoic acid, was used as a stable carbene precursor to react with
Si–H terminated porous silicon (PSi) under microwave irradia-
tion. After formation of a molecular monolayer, the end carboxyl
group was converted to an amine-reactive crosslinker species,
NHS ester, and finally to a Bovine Serum Albumin (BSA) mono-
layer on PSi.
The assembly of organic monolayers on semiconductor
surfaces by chemisorptive strategies remains an active area of
research.1 Many organic species bearing functionalities such as
alkenes, alkynes,2 alkyl halides,3 alcohols,4 aldehydes,5 carbox-
ylic acids,6 acid chloride,7 Grignard reagents,8 and alkyllithium
reagents9 had been used to assemble organic monolayers on
silicon surfaces through the robust Si–C or Si–O–C bonds by
heating, photo irradiation,10 microwave irradiation,11 electro-
chemical reduction, catalysis, or other methods. The reaction
mechanism is commonly accepted as a surface-propagated chain
reaction in which an alkyl radical formed by the addition of
organic functionalities to a surface silicon radical abstracts a
hydrogen atom from an adjacent silicon hydride. To our knowl-
edge, the typical carbene intermediates from diazirines have
been employed for grafting biomolecules on silicon oxide and
diamond substrates,12,13 but not on hydrogen-terminated silicon
surfaces. Similar examples include: 1) diazirine diazomethane14
under a 365-nm light irradiation forms a linear alkane monolayer
on silicon through a singlet methylene intermediate and 2)
benzenediazonium salts15 under an electrochemical reduction
generate aryl radicals to react with Si–H for coupling organic
monolayers.
Herein, we report the assembly of a carboxyl-terminated
organic molecule on porous silicon (PSi) from a diazirine
species in order to graft biomolecules (Scheme 1). In view that
the carboxyl terminal monolayers possess versatile chemical
possibility to immobilize biomolecules, in particular proteins,
a bifunctional species, diazirine- and carboxyl-functionalized
crosslinker, 1,4-(1-azi-2,2,2-trifluoroethyl)benzoic acid (labeled
as H in Supporting Information Scheme S1), was synthesized
and self-assembled on PSi under microwave irradiation. The
The grafting procedure in Scheme 1 was monitored by FTIR
and XPS analyses. The IR spectra of each surface after stepwise
reactions were recorded in Figure 1. The spectrum of a fresh hy-
dride-terminated PSi (a) exhibits typical tripartite bands around
2100 cmꢁ1, contributed from Si–Hx (x ¼ 1{3) stretching modes
(2084 from ꢀSi–H1, 2109 from ꢀSi–H2, and 2130 cmꢁ1 from
ꢀSi–H3). The Si–Hx bending modes exhibit the absorption peaks
at 906, 660, and 623 cmꢁ1 respectively. After reaction with a di-
azirine compound H, 4-(1-azi-2,2,2-trifluoroethyl)benzoic acid,
under the microwave irradiation for 20 min, surface b was ob-
tained, and its spectrum in Figure 1b exhibits distinctive differ-
ence from a with the appearance of a broad IR band of oxidized
silicon at 1087 cmꢁ1 and organic groups. The typical stretching
band of the carboxylic acid at 1696 cmꢁ1 indicates the existence
of free benzoic acid functionalities. The skeleton vibration of
benzene also appears at 1610, 1510, and 1415 cmꢁ1. The peak
at 1255 cmꢁ1 can be attributed to the CF3 stretching vibration.
However, the weak band at 2923 cmꢁ1 from alkyl CH2 should
be attributed to the surface contamination of hydrocarbons from
the lab atmosphere. The characteristic tripartite bands of Si–Hx
from a evolve into two broad bands at 2098 and 2248 cmꢁ1
.
According to the calculation of Lucovsky,16 the peak 2248
cmꢁ1 is assigned to the oxygen back-bonded Si–H species,
(O2)SiH2 and (O3)SiH. These oxygen back-bonded Si–H species
also show broad bending modes at 867 and 626 cmꢁ1. The peak
2111 cmꢁ1 is assigned to the unreacted residues of Si–H species.
Surface c was obtained by activation of b with NHS/DCC, its IR
a
b
c
O
CF
O
C
3
H
CF
3
O
C
O
H
C
2
3
1
C
O
CH
N
O
BSA
OH
C
N
H
H
d
H
CF
3
a
b
c
d
0.2
Scheme 1. Surface modification of porous silicon. 1,4-(1-
Azi-2,2,2-trifluoroethyl)benzoic acid (H) in p-xylene under
microwave irradiation; 2, NHS/DCC in 1,4-dioxane; 3, BSA
in PBS at pH 9.0; a, H-terminated porous silicon; b, carboxyl-
terminated PSi; c, NHS ester-terminated PSi; d, protein BSA
pendant PSi.
3000
2500
2000
1500
1000
500
Wavelength / cm-1
Figure 1. Transmittance FTIR spectra of surfaces a, b, c, and d.
Copyright Ó 2006 The Chemical Society of Japan