Desorption/ionization on self-assembled monolayer surfaces (DIAMS)

Full text of DOI:10.1002/jms.1036

JOURNAL OF MASS SPECTROMETRY
J. Mass Spectrom. 2006; 41: 830–833
Published online 22 May 2006 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jms.1036
JMS Letters
Received 30 January 2006; Accepted 3 February 2006
Dear Sir,
compounds.14 The latter was named surface-enhanced neat desorption
(SEND) and has been mainly compared to the DIOS method in terms
of the photon-absorbing surface.
By analogy with the surface properties of a UV-absorbing
semiconductor (such as porous silicon), and with regard to the
analytical flexibility offered by the possibility of functionalization
of activated surfaces, we propose here the use of some organic
surfaces in the LDI techniques. This new free-matrix laser desorption
ionization method developed is termed desorption/ionization on
self-assembled monolayer surfaces (DIAMS) and uses self-assembled
monolayers (SAMs). SAMs are defined as two-dimensional films,
one molecule thick, covalently assembled at an interface. These
organic assemblies result, in most cases, from the reaction in solution
between the headgroup function of a molecular constituent and a
metal, oxide or semiconductor surface.15 For the present work, the
adsorbates fixed onto a gold surface by a headgroup had to be
designed to organize themselves spontaneously on a crystalline or
semicrystalline structure, namely, by van der Waals interaction,
hydrogen bonds, etc. We have focused our attention on alkanethiols
covalently bonded to a redox chromophore. The latter has to absorb
at the laser wavelength (i.e. 337 nm) and has to maintain the
conductivity of the sample–metal interface of the MALDI plates
of the mass spectrometer. The redox chromophore used in this
work is the 5,50-disubstitued-2,20-bithiophene. The redox properties
of the chromophore allow for a simple and quick characterization
of the SAMs by electrochemistry. An alkane-thiol of ten carbons
has been used as a linker to the gold surface. This number gives
a better thermodynamic stability to the organosulfur adsorbates on
the gold surface and better organization than a shorter linker.15 The
introduction of the methyl group as terminal function allows the
formation of a hydrophobic surface and avoids the possibility of
polymerization of the bithiophene unit.
Desorption/ionization on self-assembled monolayer surfaces
(DIAMS)
In mass spectrometry, the laser desorption/ionization (LDI) method
refers to an overall process by which the energy absorption of a
laser beam by a localized region of an irradiated surface leads to
the emission of gaseous charged particles. The most important steps
involve the rapid dissipation of energy followed by the vaporization
of the analyte, which acquires translational energy. Therefore, the
direct irradiation of a sample, which induces a very rapid local
heating and the absorption in the UV or IR region by the analyte,
reduces the ionization efficiency and activates the dissociation of
weak bonds during the energy transfer. This constitutes a limitation
of the LDI technique, since only fragmented ions are detected even
in the case of small-molecule analysis.
To circumvent this problem, Karas and Hillenkamp1 and
Tanaka2 have developed the matrix assisted laser desorp-
tion/ionization (MALDI) technique. This method uses a photon-
absorbing mediator, i.e. an aromatic matrix molecule, which is
co-crystallized with the analyte. The analyte dispersion into the
matrix crystals and the structure of the irradiated matrix crystal
surface play a key role in the energy dissipation process.3,4 That
leads to the vaporization of the matrix and the release of the analyte
molecules as ions in the gas phase. In the MALDI mass spectra, the
detected charged species are either preformed ions that are directly
desorbed, or vaporized neutral analytes undergoing gas-phase ion-
ization by the matrix–analyte reaction.5 The use of such ‘softened’
ablation/ionization processes has allowed MALDI to be considered
as a powerful mass spectrometric method for biochemical analy-
sis because of the detection of ions from high-molecular-weight
molecules (M.W. > 5000 u).6
The synthesis of the SAMs precursor compounds starts from
the 2-bromothiophene and is described in Scheme 1. Because of the
propensity of the thiol function to oxidation, all characterizations
in solution were carried out on the protected alkane-thiol (1).
As expected from previous results reported in the literature, the
characteristic optical band of 1 is observed at 340 nm.16 The redox
behavior of 1 was investigated by cyclic voltammetry (CV) in a
glove box containing dry, oxygen-free (<1 ppm) argon at room
temperature. CV of 1 in 0.2 M TBAHP/CH3CN exhibits a reversible
one-electron oxidation wave at 0.49 V (vs FcC/Fc).17
The UV irradiation of the matrix produces a wide range
of ions, which lead to high-intensity background signals in the
low-mass range. In addition, the sample preparation involving co-
crystallization of the guest and a matrix (with a molar ratio of
1000–10 000) is a matter of trial and error since a good matrix for a
particular sample must be found.
In order to overcome these limitations, free-matrix laser
ablation/ionization methods have been proposed in the literature.
The LDI on porous silicon (DIOS) technique was developed because
of the structure of the surface, which in this case provides a
scaffold for retaining solvent and analyte molecules, and the UV
absorptivity of the substrate, which affords a conversion mode
for the transfer of the photon energy to the translational energy,
leads to the sample vaporization process.7 The ‘surface-assisted
laser desorption/ionization’ (SALDI) method uses a thin layer of
activated carbon particles immobilized on an aluminum support.8
In this case, the sample can be directly deposited on the surface
plate. An alternative technique uses a suspension of a fine graphite
powder or functionalized nanoparticles9 in a solution of the analyte
in an organic solvent. Peptides and organic compounds were then
detected in DIOS and SALDI mass spectrometry as protonated
molecules and/or as alkali metal adducts.10,11 The functionalization
concept of the mass spectrometric probe surface has been previously
proposed through the pioneering work of Hutchen and Yip, and
has promoted the development of the surface-enhanced laser
desorption/ionization (SELDI) method.12 In this case, the chemical
and biochemical protein chips array surfaces play a role, namely, for
the extraction and/or concentration of the sample prior to depositing
a matrix solution on the derivatized surface.13 An alternative method
consists also in attaching directly a matrix molecule, such as an
aromatic thiol derivative, on a gold surface without any chemical
modification between the metal surface and the energy-absorbing
The SAMs were obtained by immersing freshly prepared Au
electrodes (typically, an adhesion layer of chrome of ca 2 nm,
followed by a layer of gold of ca 35 nm, deposited by physical
vapor deposition on a glass support) for 48–96 h in a solution of
alkane-thiol (2) (dichloromethane, 1 ð 10ꢀ3 M). All experiments were
ŁCorrespondence to: E. Levillain and D. Rondeau, Laboratoire de
Spectrome´trie de Masse – Service Commun d’Analyses
Spectroscopiques – Universite´ d’Angers, 2 Bd Lavoisier, 49045
Angers, France. E-mail: David.rondeau@univ-angers.fr
Figure 1. Cyclic voltammetry of the SAMs of 2 in 0.2 M
TBAHFP/CH2Cl2.
Copyright  2006 John Wiley & Sons, Ltd.

JMS letters 831
S
S
ii)
iii)
Br
i)
S
S
NC
S
S
NC
CN
Me
S
S
S
S
S
iv)
Me
HS CH
S
v)
S
S
S
2
Me
S
CH
2
S
Me
S
O
S
S
(1)
(2)
Scheme 1. Synthesis of alkane-thiol (2). Reagents and conditions: (i) Magnesium, NidpppCl2, Et2O, reflux; (ii) nBuLi, sulphur and
2-bromopropionitrile, THF, room temperature; (iii) Cesium hydroxide (1 equiv.) and methyl iodide, DMF/MeOH, room temperature,
2 h;18 (iv) Cesium hydroxide (2 equiv.) and 10-thioacetate-bromodecane, DMF/MeOH, room temperature, overnight; (v) Cesium
hydroxide, THF, room temperature, 3 h.
Figure 2. (a) Positive ion DIAMS mass spectrum of two poly(ethyleneglycol) mixtures with nominal molecular weights centered
around 400 and 600. The peaks of NaC (m/z 23) and KC m/z 39 are depicted in the mass spectrum with the alkali metal adducts of
PEG, i.e. [M C Na]C and [M C K]C. (b) Expansion of the positive ion DIAMS mass spectrum of a peptide mixture. Signals at m/z
578.2, m/z 1046.5 and m/z 1347.6 correspond to the [M C Na]C of the leucine enkephaline (2 ð 10ꢀ10 mol deposited on the plate),
the [M C H]C of the angiotensine II (8 ð 10ꢀ11 mol deposited on the plate) and the [M C H]C of the substance P (8 ð 10ꢀ11 mol
deposited on the plate), respectively.
performed in a glove box at room temperature. The characteristics
of the SAMs were as expected. Firstly, as shown Fig. 1, the
electrochemical response of the SAMs of (2) is consistent with a
reversible redox system (C0.49 V (vs FcC/Fc)) confined at the Au
electrode, as shown by the linearity of peak currents with the scan
rate (up to 50 V sꢀ1). Secondly, the surface coverage () reaches a
value of 2 (š0.4) ð10ꢀ10 mol cmꢀ2. This reproducible value shows a
good organization.15 Finally, the SAMs show excellent stability over
several weeks.
In order to test the ability of DIAMS in the field of application
of free-matrix laser ablation/ionization mass spectrometry, samples
such as poly(ethylene glycols) (PEGs), peptides, fatty acids and
glyceride mixtures were directly spotted on the organic surface. MS
analyses were performed on a Bruker Biflex III time-of-flight (TOF)
mass spectrometer (Bruker-Daltonic, Bremen Germany) equipped
with a Scout 384 probe ion source. Samples were directly deposited
on the gold–SAMs interface and desorbed/ionized from the probe
tip with a 337-nm pulsed nitrogen laser (model VSL-337i, Laser
Science Inc., Boston, MA) having a repetition rate of 3 Hz and a
pulse width of 2 ns. All analyzed samples were purchased from
Aldrich. Positive-ion mode mass spectrum of PEGs (Fig. 2(a)) shows
NaC and KC abundant ions together with the alkali adducts of
the PEGs. Although some relatively low-intensity ions between
m/z 39 and the first PEG ions (m/z 305) appear on the DIAMS
mass spectrum of Fig. 2, no signals are observed from the SAMs
themselves. Peptide analysis shows that DIAMS can produce quasi-
molecular [M C H]C ions or even [M C Na]C ions if the peptide does
not contain basic amino acids such as leucine enkephaline (Fig. 2(b)).
DIAMS can be mainly suitable for lipids analysis since glyceride can
be detected in the positive-ion mode through the [M C Na]C ions
(Fig. 3(a)), whereas the fatty acids are observed in the negative-ion
mode as deprotonated species (Fig. 3(b)), and this, without prior
sample treatment. Note that in the negative-ion mode no gold or
gold-thiolates emission is observed in the DIAMS mass spectrum of
Fig. 3(b).14,19
In conclusion, the results show that DIAMS can be considered
as a novel free-matrix LDI method for generating gas-phase ions
directly from samples directly deposited onto a self-assembled
Copyright  2006 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2006; 41: 830–833
DOI: 10.1002/jms

832
JMS letters
Figure 3. (a) Expansion of the positive-ion DIAMS mass spectrum of a mixture of ˛,ꢀ-dihexadecanoin [M C Na]C m/z 591.4,
tritridecanoin [M C Na]C m/z 703.6 and trihexadecanoin [M C Na]C m/z 829.7. (b) Negative-ion DIAMS mass spectrum of a fatty acid
mixture. Signals at m/z 228, m/z 255, m/z 283, m/z 311, m/z 339 and m/z 367 correspond to the [M ꢀ H]ꢀ of the tetradecanoic,
hexadecanoic, octadecanoic, eicosanoic, docosanoic and tetracosanoic acids, respectively (5 ð 10ꢀ10 mol of each analyte was
deposited on the plate).
monolayer surface. The analytical versatility and the ease of use of
this technique are demonstrated through the studied samples, which
encompass small peptides, natural organic compounds and synthetic
polymers. In particular, the potential of this DIAMS-MS method
appears in the case of lipids analysis. Therefore, one can consider
that the chemical modifications of SAMs may induce other surface
properties and can lead to numerous applications. For example,
in this paper, the nature of the synthetized SAMs has allowed a
hydrophobic target suitable for analyses in a lipidomic context to be
obtained, even though the detection of peptides was also possible.
We envisage the design of new SAMs that may induce different
surface properties with more hydrophilic spacers and a specific
terminal functional group that should improve the ionization yield
and allow molecular recognition processes for specific analyses.
A convenient engineering could also increase the spot concentration
and therefore the sensitivity of detection.
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Yours,
1
L. SANGUINET,1 O. ALEVEQUE, P. BLANCHARD,1 M. DIAS,1
´
ˆ
E. LEVILLAIN1Ł and D. RONDEAU2Ł
Chimie, Inge´nierie Mole´culaire et Mate´riaux d’Angers, UMR CNRS
1
6200 – Universite´ d’Angers, 2 Bd Lavoisier, 49045 Angers, France.
2
Laboratoire de Spectrome´trie de Masse – Service Commun d’Analyses
Spectroscopiques – Universite´ d’Angers, 2 Bd Lavoisier, 49045 Angers, France.
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Copyright  2006 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2006; 41: 830–833
DOI: 10.1002/jms