Two-dimensional metal-organic framework nanosheets as a matrix for laser desorption/ionization of small molecules and monitoring enzymatic reactions at high salt concentrations

Article abstract of DOI:10.1039/c6cc07371a

Stable 2-D metal-organic framework nanosheets were utilized as a superior clean-background matrix for MALDI-TOF MS analysis of small biomolecules and pollutants in both positive and negative ion modes. The matrix could unusually afford up to 1000 mM of the salt concentrations in the monitoring of the enzymatic hydrolysis of neurotransmitter acetylcholine.

Full text of DOI:10.1039/c6cc07371a

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Two-Dimensional Metal-Organic Framework Nanosheets as
Matrix for Laser Desorption/Ionization of Small Molecules and
Monitoring Enzymatic Reactions under High Salt Concentrations
Received 00th January 20xx,
Accepted 00th January 20xx
Hai-Long Liu,† Yu-Jie Chang,† Ting Fan and Zhi-Yuan Gu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The stable 2-D metal-organic framework nanosheets was utilized as silicon (DIOS),6 quantum dots,7 gold nanoparticles,8
superior clean-background matrix for MALDI-TOF MS analysis of mesoporous carbons,9 graphene,10 graphitic carbon nitride,11
small biomolecules and pollutants in both positive and negative ion proton sponges,12 and organic polymers.13 Although the high
modes. The matrix could unusually afford up to 1000 mM of the salt surface area and suitable light absorption makes MOFs as
concentrations in the monitoring enzymatic hydrolysis of promising matrix, very few reports published for the MOF
neurotransmitter acetylcholine.
matrix MALDI-TOF analysis due to the inhomogeneous bulk
nature.14 To the best of our knowledge, no 2-D MOF nanosheets
was explored as matrix for MALDI TOF MS analysis.
Metal-organic frameworks (MOFs) are porous solid-state
materials with unique properties such as ultrahigh porosity,
large surface areas, and tunable pore sizes.1 Two dimensional
(2-D) nanosheets, such as graphene, transition metal
dichalcogenides, thin layered metal oxides, and phosphorene,
are the new generation of nanomaterials due to their excellent
electronic and photonic properties, good solubility, large
accessible functional surface, and single-atom/molecule
activity.2 With the combination of the features of MOFs and 2-
D nanosheets, the 2-D MOF nanosheets are of great interest as
tools in the applications of separation, sensing and energy
storage.3
Matrix-assisted laser desorption/ionization time-of-flight
mass spectrometry (MALDI-TOF MS) is a soft ionization method
for protein analysis.4 However, the utilization of this technology
to the screening of biomarkers, drugs, and pollutant molecules
is obscured by the its limitation for the analysis of small
molecules (<700 Da). Due to the interference from the
conventional organic matrix background ions, the analysis of
small molecules on MALDI-TOF MS not only show the bad
signal-to-noise ratio but also the poor reproducibility,
originating from the matrix self-decomposition.5 To overcome
these limitations, a variety of methodology based new matrices
have been reported, such as desorption/ionization on porous
Here we successfully applied a water-stable 2-D MOF
nanosheets, namely Zn2(bim)4 (bim=benzimidazole) as matrices
for MALDI-TOF analysis of diverse molecules (Fig. 1e, 1f, 2, 4,
S4a, and S4b), confirming the efficient photon absorption and
target ionization via 2-D MOF nanosheets matrix. Further
applications confirm the ultrahigh tolerance of salt
concentrations by this matrix, which is then applied to monitor
the enzymatic hydrolysis of neurotransmitter acetylcholine.
The 2-D Zn2(bim)4 nanosheets was obtained by ultrasonic
exfoliation of pristine Zn2(bim)4 MOF (Fig. 1d),3g which was
characterized and confirmed by XRD and SEM (Fig. S1).
Although the size of nanosheets spans from dozens of to
hundreds of nanometers (Fig. 1a), the thickness of ~1 nm was
obtained using AFM, which is in agreement to its theoretical
thickness of single layer (Fig. S2). Comparing to its precursor,
the 2-D Zn2(bim)4 nanosheets gave better homogeneous
dispersion in water with strong Tyndall effect, confirming its
nanomaterial feature (Fig. 1c). The 2-D Zn2(bim)4 nanosheets
are built from coordination between zinc and benzimidazole,
which is quite stable in water referring to the hard-soft acid-
base (HSAB) theory. Thus, the homogenicity and stability made
2-D Zn2(bim)4 nanosheets as good candidate matrix for MALDI-
TOF.
In order to evaluate the performance of 2-D Zn2(bim)4
nanosheets as an efficient matrix for small molecules, a mixture
of three amino acids containing glutamic acid (Glu), histidine
(His), and tryptophan (Trp) and a nucleobases mixture
containing uracil (U), thymine (T), adenine (A), and guanine (G)
College of Chemistry and Materials Science, Jiangsu Key Laboratory of
Biofunctional Materials, College of Life Sciences, Nanjing Normal University,
Nanjing, 210023 China, E-mail: guzhiyuan@njnu.edu.cn
† H.-L. Liu and Y.-J. Chang contribute equally to this work.
Electronic Supplementary Information (ESI) available: See DOI:
10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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nanosheets matrix (Fig. 1e, Fig. S3a and Table 1). Similar results
View Article Online
DOI: 10.1039/C6CC07371A
were also observed for the analysis of nucleobases (Fig. S4a,
Table 1. MALDI-TOF Analysis of Three Amino Acids in Positive and Negative Ion Modes
with the Matrices of 2-D MOF Nanosheets, CHCA and three MOFs.[a]
MOFs
2-D
Detected
ions
m/z
Zn2(bim)4
Nanosheets
CHCA
Pristine
MIL-
ZIF-8
Zn2(bim)4
100(Fe)
148
170
186
156
178
194
227
243
146
154
203
[Glu+H]+
[Glu+Na]+
[Glu+K]+
[His+H]+
[His+Na]+
[His+K]+
[Trp+Na]+
[Trp+K]+
[Glu-H]
[His-H]
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
7
*
*
*
*
*
*
*
7
*
*
*
*
*
[Trp-H]
*
*
*
Identified ions[b]
11
11
10
Missing rate[c]
0%
2
36%
12
0%
12
9%
36
36%
5
Background
interference
ions[d]
+

0
2
2
17
0
Fig. 1 (a) TEM image of 2-D MOF nanosheets. (b) photograph of pristine Zn2(bim)4 MOF
aqueous suspension. (c) photograph of 2-D MOF nanosheets colloidal suspension. (d)
scheme of the liquid exfoliation process. 2-D Zn2(bim)4 nanosheets assisted laser
desorption ionization mass spectra of three amino acids (Glu, His and Trp) in positive-ion
(e) and negative-ion modes (f). Background interference ions were shown with red stars.
[a]
[b]
The black stars in the table mark identified ions with specific matrices.
Identified ions mean the number of identified ions. [c] Missing rate was calculated
[d]
as the ratio of missing ions to total ions. Background interference ions were
foreign ions with S/N >6.
were chosen as model analytes. The peaks for Glu (148, [M+H]+;
170, [M+Na]+; 186, [M+K]+), His (156, [M+H]+; 178, [M+Na]+; 194,
[M+K]+) and Trp (227, [M+Na]+; 243, [M+K]+ ) are all well
detected with the matrix of 2-D Zn2(bim)4 nanosheets in
positive-ion mode (Fig. 1e). Under negative-ion mode, the
deprotonated ions for amino acids were all clearly shown in Fig.
1f. Similarly, the protonated and sodium or potassium adduct
ions as well as the deprotonated ions for nucleobases were also
clearly observed with the matrix of 2-D Zn2(bim)4 nanosheets
(Fig. S4a and S4b). In contrast, when CHCA as the matrix, there
were no Glu peaks, Trp potassium adduct peak, or U peaks (Fig.
S3a, Fig. S4c, Table 1 and S2). The missing rates of CHCA are 36%
(Table 1) and 27% (Table S2) for amino acids and nucleobases
analysis, respectively, while no ion was missing for 2-D
Zn2(bim)4 nanosheets matrix. All data were obtained under the
optimized laser intensity and matrix/analyte molar ratio (Fig.
S5).
The 2-D Zn2(bim)4 nanosheets not only show the high
coverage for the desired ions but also demonstrate the clean
background and high signal to noise ratio (S/N). The background
interference ions (impurities with S/N > 6) were rarely observed
with the matrix of 2-D Zn2(bim)4 nanosheets. In contrast, severe
background interference ions were observed when CHCA as the
matrix due to its fragmentation. For the amino acids in the
positive-ion mode, 12 background interference ions were
detected with CHCA, while only 2 observed for 2-D Zn2(bim)4
S4c and Table S2). It is worth noting that there are no
background interference ions with negative-ion mode for both
amino acids and nucleobases (Fig. 1f and S4b). Meanwhile,
significant background interference ions were shown in
negative-ion MS spectrum with CHCA matrix (Fig. S3b and S4d)
although clear peaks and lower low detection limit were
obtained with CHCA matrix (Table S3).
To make an impartial comparison and explore the potential
advantages of 2-D Zn2(bim)4 nanosheets, two of the reported
MOF matrices (ZIF-8 and MIL-100(Fe), Fig. S6 and S7) and the
pristine Zn2(bim)4 MOF were selected as matrices (Fig. S8, S9,
Table 1, and S2). MIL-100(Fe) and ZIF-8 were both reported as
efficient matrices for small molecules, such as pollutants and
biomolecules.14 With the nanosized feature, ZIF-8 exhibited
good reproducibility in negative-ion mode.14c Our 2-D Zn2(bim)4
nanosheets were single-layered nanosheets built with stable
coordination zinc-benzimidazole bond. Structurally, 2-D MOF
nanosheets offer suitable particle size, large accessible surface,
and stable structure, rendering homogenicity in suspension,
lower missing rate for positive ion species (Table 1 and Table S2),
clean background and low detection limit feature (Fig. 1e, 1f,
S4a, S4b, S10, Table 1, S2 and S3). The better coverage for dual
ion modes with the matrix of 2-D nanosheets possibly
originated from the indiscriminative absorption onto the
accessible surface of the 2-D nanosheets. It is worth noting that,
2-D MOF nanosheets also clearly demonstrate better coverage
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and lower background interference ions comparing with the the stable nature of 2-D Zn2(bim)4 nanosheets, encouraged us
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DOI: 10.1039/C6CC07371A
to explore more analytical performance. Reproducibility for
pristine Zn2(bim)4 MOF (Table S2).
Fig. 4 The enzyme catalyzed conversion from acetylcholine to choline monitored by the
positive ion mode with 2-D Zn2(bim)4 nanosheets as the matrix using MALDI-TOF MS. The
reaction kinetics was shown by the changes of acetylcholine ion (m/z 146) intensities
over time. The inset MS spectrum obtained at 6 min shows both the substrate of
acetylcholine ion (m/z 146) and the product of choline ion (m/z 104).
MALDI-TOF MS largely depends on the ionization step, where
the homogeneous mixture of solid state with analytes is highly
desired. Thus, well-dispersed nanosheets will enhance the
reproducibility. Here, we show the MS signal for ten parallel
spots with the matrices of 2-D Zn2(bim)4 nanosheets, CHCA and
other three MOFs (Fig. S13 and S14). The relative deviation of
nanosheet data is significantly smaller than any other four
matrices. This homogeneous feature was supported by matrix
pictures after crystallization (Fig. S15) and in consistent to the
obtained SEM and TEM pictures.
Fig. 2 MALDI-TOF mass spectra of serotonin ([M+H]+, m/z 177; [M-H]-, 175), testosterone
([M+Na]+, m/z 311; [M+K]+, m/z 327; [M-H]-, m/z 287) and bisphenol A ([M+Na]+, m/z
251; [M+K]+, m/z 267; [M-H]-, m/z 227) with a matrix of 2-D Zn2(bim)4 nanosheets in
positive-ion and negative-ion modes.
Encouraged by the above results, the Zn2(bim)4 nanosheets
matrix was further explored with other analytes, including
serotonin, testosterone and bisphenol A (BPA). The chemicals
are crucial in pharmaceutical treatments of depression, the
prognosis of disease and the safety of environment.15 In both
the positive and negative ion modes, the serotonin (177, [M+H]+;
175, [M-H]-), testosterone (311, [M+Na]+; 327, [M+K]+; 287, [M-
H]-) and BPA (251, [M+Na]+; 267, [M+K]+; 227, [M-H]-) are all
detected with the matrix of 2-D Zn2(bim)4 nanosheets (Fig. 2).
The significant signals with moderate background interference
ions were obtained in positive-ion mode, while, in negative ion
mode, high S/N ratio was observed. The 2-D Zn2(bim)4
nanosheets with large surface area could adsorb the analytes
and thus prone to form a uniform spots on the sample plates to
prevent the detachment of the matrix under vacuum.
Fig. 3 Mass spectra of 10 mM histidine (*, [His+H]+, m/z 156; &, [His+Na]+, m/z 178; #,
[His+K]+, m/z 194; *, [His-H], m/z 154) with 2-D Zn2(bim)4 nanosheets as matrix from 0
to 1000 mM of NH4HCO3 in both positive (a) and negative (b) ion modes.
The efficiency of 2-D MOF nanosheets was further explored
to monitor the hydrolysis of acetylcholine (ACh) by
acetylcholinesterase (AChE) in ammonium bicarbonate buffer.
ACh is an important neurotransmitter and hydrolyzed by AChE
at chemical synapse in neuromuscular junction, where the
hydrolysis of ACh by AChE serves to terminate synaptic
transmission. Although the activity of AChE is pretty hard to
measure by the conventional colorimetric methods, the
monitoring hydrolysis of ACh by AChE is of great importance to
The stable 2-D Zn2(bim)4 nanosheets with large accessible
surface showed clean background and low missing percentage
in dual-ion modes, which is superior to the conventional CHCA
and several MOF matrices. To further confirm the stability, 2-D
Zn2(bim)4 nanosheets suspension was tested as matrices after
suspension for 1 day, 4 days, and 8 days. The MS signal
suggested that suspension of 2-D Zn2(bim)4 nanosheets could
serve as matrix for continuous 8 days without significant signal
reduction (Fig. S11 and S12). This promising data benefits from
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tolerance of 2-D Zn2(bim)4 nanosheets matrix was evaluated
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unusually afford up to 1000 mM of NaCl or NH4HCO3 in both
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In conclusion, the stable 2-D Zn2(bim)4 nanosheets were
utilized as efficient matrix for MALDI-TOF MS analysis of amino
acids, nucleobases, neurotransmitters, hormones and pollutant
molecules. The clean background MS spectra and low missing
rates was obtained in dual-ion modes, which is superior to the
conventional CHCA and several MOF matrices. Stable and
reproducible MS spectra was obtained in both positive and
negative ion modes. The matrix could unusually afford 1000
mM of the salt concentrations for NaCl and NH4HCO3 and was
successfully employed in the monitoring of the activity of
acetylcholinesterase for the hydrolysis of neurotransmitter
acetylcholine.
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