A Plasticizer-Free Miniaturized Optical Ion Sensing Platform with

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A Plasticizer-Free Miniaturized Optical Ion Sensing Platform with Ionophores and Silicon-based Particles Xinfeng Du, Liyuan Yang, Wenchang Hu, Renjie Wang, Jingying Zhai, and Xiaojiang Xie Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00360 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Analytical Chemistry

A Plasticizer-Free Miniaturized Optical Ion Sensing Platform with Ionophores and Silicon-based Particles Xinfeng Du#, Liyuan Yang#, WenChang Hu, Renjie Wang, Jingying Zhai, and Xiaojiang Xie* Department of Chemistry, Southern University of Science and Technology, 518055, Shenzhen, China Email: [email protected] ABSTRACT: Nanoscale ionophore-based ion-selective optodes (nano-ISOs) are effective sensing tools for in situ and real time measurements of ion concentrations in biological and environmental samples. While searching for novel sensing materials, nano-ISOs free of plasticizers are particularly important for biological and environmental applications. This work described plasticizer-free nano-ISOs based on Si-containing particles including PEGylated organosilica nanoparticle, PDMS nanospheres and SiO2 microspheres, with diameters around 50 nm, 100 nm, and 5 µm, respectively. The platform enabled the use of highly selective ionophores, where the nano-matrices played important roles in tuning the ion-carrier complex formation constants and led to better selectivity for the PEGylated organosilica nano-ISOs than those based on PDMS. Using the versatile silica chemistry, pH and ion dual sensing was achieved on SiO2 microspheres. In addition, increasing the cross-linking degree of the PDMS nano-ISOs extended the linear response range and cellular uptake experiments showed that the nano-ISOs could readily enter HeLa cells with very low cytotoxicity.

As the optical counterpart of ion-selective electrodes

fluorescent nanoparticles containing conjugated polymers as

(ISEs), ionophore-based ion-selective optodes (ISOs) are

both matrix material and signal transducer.19 Clark and

valuable analytical tools for the optical detection and mapping

co-workers showed the in vivo photoacoustic monitoring of

of a number of ions including alkali and alkali earth metal

lithium

1-3

ions,

4-6

transitional metal ions,

anions,7-9

polyions.10,11

various main group inorganic

ion

concentrations 22

using

lipid-stabilized

PVC

+

nanoparticles. Nanosensors for K made by polymerizing and

an

cross-linking nBA and HDDA were reported by Hall and

ion-exchanger, a H chromoionophore, and an ionophore for the

co-workers.21 Ultrasmall Pluronic F-127 based ion-selective

target analyte in a lipophilic matrix such as plasticized poly

nanospheres were reported by Bakker and co-workers.23 Based

(vinyl chloride) (PVC). Recently, the miniaturization of ISOs

on this platform, our group recently reported the detection of

injected new vigor and vitality to the field thanks to a series of

blood potassium by embedding the nanospheres in agarose

new sensing possibilities at the micrometer and nanometer

hydrogel.24 In addition to Pluronic F-127, other poloxamers

and

ISOs

typically

contain

+

12-16

scale.

The sensor matrices continued their important roles

were also recently used to make nano-ISOs for photoacoustic

not only in acting as a host for the sensing components but also

and fluorescence imaging of potassium ions.25 Note that in most

in influencing the sensor response and the compatibility in the

of the existing nano-ISOs, the plasticizer bis(2-ethylhexyl)

intended application. For the latter, plasticizer-free ISOs are

sebacate (DOS) remains frequently used. According to the MTT

preferred because of the potential leakage and toxicity of the

assay by Jiang and co-workers, cellular uptake of nanosensors

plasticizers, which is well-known to those familiar with

containing DOS at relative high concentration could be quite

membrane based ISEs.

toxic.26

Nanoscale

ionophore-based

ion-selective

optodes

As for the micro-scale ISOs, Bakker and co-workers

(nano-ISOs) have attracted lots of attention in recent years.

demonstrated the mass production of micro-sensing particles

Among others, the groups of Kopelman,17 Bakker, Clark,18

using a sonication-based particle caster.27 Hall and co-workers

Michalska,19 and Hall20,21 have contributed different protocols to

showed

+

that

organosilica

microcapsules

containing

fabricate nano-ISOs. In particular, K PEBBLEs with 500-600

chromoionophores could reduce the response time by two

nm in diameter were reported by Kopelman and co-workers

orders of magnitude than the solid ones.28 Later on, surface

using decyl methacrylate-co-hexanediol dimethacrylate to host

adsorption of the sensing ingredients on polystyrene beads

the sensor components.17 Michalska and coworkers reported

provided another strategy to fabricate ISOs at the microscale.29

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Page 2 of 9

Due to the very slow diffusion in polystyrene, the modifications

2-(N-Morpholino)ethanesulfonic acid (Mes), and agarose with

were restricted on the surface region and the sensor response

low gelling temperature were obtained from Sigma−Aldrich®.

exhibited a wide linear range.

Methoxy-PEG-silane (molecular weight 5000) was from

To use what material as the matrix of the nanosensors

ZZStandard®, Shanghai, China. Lumogen red was obtained

indeed requires lots of attention with at least the following three

from BASF®. Silica microspheres with average diameter of ca.

criteria to be considered. 1. The particle material should contain

5 µm were obtained from Bangs Laboratories, Inc.. Sylgard 184

lipophilic parts because the sensor components such as

base and the curing agents were purchased from Dow Cornings.

ionophores are generally lipophilic. 2. The microenvironment

The DMEM cell culture medium (with 4.5 g/L glucose), sterile

should be preferentially nonpolar to ensure sufficiently high

phosphate buffered saline (PBS), fetal bovine serum (sterile

ion-carrier complex formation constants for good selectivity. 3.

filtered), penicillin streptomycin solution, and 0.25 % trypsin

The material of the particles should be compatible with the

containing 2.21 mM EDTA were obtained from Corning, Inc.

intended application. Characteristics including material choices

in China. All salts used were at least analytical grade. All

and preparation methods of several previously reported

aqueous solutions were prepared by dissolving the appropriate

ionophore-based

salts in water purified by Milli-Q Integral 5. Cell Counting Kit-8

microsensors

and

nanosensors

were

summarized in Table S1 in the supporting information. Previous

(CCK-8) was purchased from MedChem Express® in China.

methods based on self-assemble (also known as precipitation in

Modification of silica microspheres. To a mixture of

mixed solvents) mainly resulted in non-crosslinked particles.

ethanol/ H2O (7:1 v/v, 24 mL) in a round-bottomed flask were

18,23,25,30,31

To make cross-linked nanosensors, free-radical

polymerization has been adopted.17,21

added 11 mg of SiO2 microspheres, 12 mg of F127 , 70 µL of 3-aminopropyltriethoxysilane and 300 µL of NH3· H2O. The

Here, we report plasticizer-free nano-ISOs based on

reaction mixture was stirred at room temperature for 4 h

Si-containing nanomaterials including PEGylated organosilica

followed by addition of 50 mg of methoxy-PEG-silane. After

nanoparticles, polydimethylsiloxane (PDMS) nanospheres, and

reacting for another 4 h and removal of NH3· H2O, 4.8 mg of

PEGylated SiO2 microspheres. These silicon-based particles are

FITC was added into the flask. Then the reaction mixture was

generally

and

stirred at room temperature overnight and the product was

Therefore, further biological applications

purified by first centrifugation and 3 days of dialysis with

with the proposed nanosenosrs could be advantageous.

cellulose dialysis tubing from Sigma® (typical molecular

Modification on the SiO2 microspheres enabled us to make dual

weight cut-off = 14000).

sensors for pH, and potassium (or lithium) ions. The nano-ISOs

Preparation

recognized 32,33

biocompatible.

as

environmental

friendly

of

the

PEGylated

organosilica

showed tunable detection range and high selectivity. Based on

nanoparticles. Typically, trimethoxy(propyl)silane (100 µL)

established optode theory, the ion-carrier complex formation

was added to a stirring mixture composed of 12.5 mL of ethanol,

constants were obtained and compared for the different matrix

1.5 mL of H2O, 500 µL of

materials. In the PDMS nanosensor case, increasing the degree

4

of cross-linking was found to enhance the linearity of the sensor

methoxy-PEG-silane dissolved in 4 mL of ethanol was added

response

and the reaction mixture was stirred for another 4 h. Then the

h

of

reaction

at

NH3· H2O and 8 mg of F127. After room

temperature,

100

mg

of

nanoparticles were collected by centrifugation and washed 5

EXPERIMENTAL SECTION Reagents.

times with deionized water.

Trimethoxy(propyl)silane,

Preparation of the PDMS nanoparticles. Typically, 5

(F127),

mg of F127, 10 mg of Sylgard 184 base, 0.5 mg of 184 curing

ammonium hydroxide solution, fluorescein isothiocyanate

agent were dissolved to 2 mL of THF. The solution was pipetted

(FITC),

II,

and injected into 15 mL of deionized water on a vortex at 1000

sodium

rpm. After blowing compressed air on the surface of the

(NaTFPB),

suspension for 30 min, the mixture was quickly heated to 65 °C

3-aminopropyltriethoxysilane,

chromoionophore

chromoionophore

Pluronic®

I,

F-127

chromoionophore

VII,

tetrakis-[3,5-bis(trifluoromethyl)-phenyl]

borate

potassium ionophore III (BME 44), lithium ionophore VI, 2-amino-2-(hydroxymethyl)-1,3-propanediol

(Tris),

and kept for 4 h in an oil bath for the crosslinking. Sensor components incorporation. Typically, 1.8 mg of lithium ionophore VI, 1.2 mg of chromoionophore VII and 2.5


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mg of NaTFPB were dissolved in 2 mL of THF to form a

suspension of nanoparticles onto a copper grid with carbon

homogeneous solution. A volume of 200 µL of the THF solution

support films, followed by drying in air.

was pipetted and mixed with 5 mL of particle suspension.

For the FL imaging assay, HeLa cells cultured in a 48-well

Compressed air was blown onto the surface of the resulting

plate were respectively treated with afore mentioned nano-ISOs

dispersion for at least 30 min to remove THF. Similarly, for the

suspension in DMEM medium for 30 min. The suspension was

potassium-selective ISOs, the THF cocktail contained 1.9 mg of

removed carefully and the cells were washed 3 times with PBS

potassium ionophore III, 0.9 mg of chromoionophore I and 2.6

buffer (10 mM, pH 7.4). The cells were imaged using a 586 nm

mg of NaTFPB. To evaluate the pH response of the silica

LED (15 nm bandwidth) as excitation and a 20× objective lens

micro-ISOs, the THF solution contained 1.0 mg of Lumogen red.

(phase contrast). For the confocal microscopic imaging, 10 mg

For the PDMS nano-ISOs, the cocktail contained 2.1 mg of Na

of agarose was added to 1 mL of afore mentioned

TFPB, 1.6 mg of lithium ionophore VI, and 1.1 mg of ETH

potassium-selective silica microspheres. The mixture was

2439 in 2 mL of THF.

heated in a water bath until all the agarose was dissolved. Then

Instrumentation

and

measurements.

In

general,

200 µL of the liquid was pipetted into a glass petri dish and left

absorption spectra were measured using an ultraviolet−visible

to cool to room temperature. The particles were imaged under

(UV-vis) absorption spectrometer (Evolution 220, Thermo

488 nm and 561 nm excitation with 20× and 40× objective

Fisher Scientific). Fluorescence signals were measured with a

lenses.

fluorescence spectrometer (Fluorolog-3, Horiba Jobin Yvon).

To evaluate the toxicity of the nanosensors to HeLa cells,

Dynamic light scattering (DLS) were measured using a zeta

cell counting kit-8 was used according to the manual. A

sizer (Nano ZSE, Malvern). Confocal microscopic images were

calibration was first performed on different numbers of cells

obtained under a confocal microscope (A1R, Nikon) with 20×

using 10 μL of CCK-8 reagent every 100 μL of cell culture

and 40× objective lenses and using 488 nm and 561nm laser line

medium. Cells were incubated in 96 well plate (100 μL of

as excitation. The fluorescent imaging of cells were taken by a

medium per well) with different amount of nanosensors (10, 30,

microplate reader (cytation5, BioTek). Cell numbers were

and 50 μL) for 24 hours. The absorbance at 450 nm was

counted with an automated cell counter (Countess II,

evaluated on a microplate reader.

ThermoFisher). For the Li+ and K+ sensing optodes, calibration curves were measured in buffer solutions at the indicated pH (10 mM Tris-HCl for pH 7 and 10 mM Mes-NaOH for pH 5.5) values with gradual addition of KCl or LiCl stock solutions. Transmission electron microscopy (TEM) was carried out on a Hitachi HT-700 microscope operating at an acceleration voltage of 100 kV. The samples were prepared by dropping

RESULTS AND DISCUSSION Dual sensing silica micro-ISOs. Silica microspheres of ca. 5 µm in diameter were modified with 3-aminopropyl groups through the Stöber process. The amine groups were further reacted with FITC, a fluorescent pH indicator. As shown in Figure 1a, confocal laser scanning microscopy confirmed that

Figure 1. (a) Confocal fluorescence imaging of FITC modified SiO2 microspheres. Scale bar: 5 μm. (b) pH response of the FITC modified SiO2 microspheres in fluorescence mode containing Lumogen red as a reference dye. (c) Confocal fluorescence imaging of the dual sensing SiO2 microspheres incorporating ETH 5294 and surface modified with FITC. Scale bar: 5 μm. (d) Response of the K+-selective SiO2 micro-ISOs to various concentrations of ions at pH 7.0. (e) Response of the Li+-selective SiO2 micro-ISOs to various concentrations of ions at pH 7.0



Page 4 of 9

the FITC modification occurred successfully on the surface of the silica microspheres. The pH response in FITC emission intensity was shown in Figure 1b. The pH response was evaluated by comparing the FITC emission at 510 nm with a reference dye Lumogen Red (emission at 660 nm). The effective pKa of ca. 6.5 was close to the pKa value of fluorescein in water. To enable metal ion-sensing, the ISOs components were dissolved in THF and adsorbed onto the microspheres through a mixed solvent method (see experimental section for details). Figure 1c shows the confocal fluorescence image overlay for the FITC emission in the green channel and the emission from the

unevenly in the microspheres due to the mesoporous structure.

Figure 2. TEM imaging of the PEGylated organosilica nanoparticles (a) and the PDMS nanoparticles (b). (c) Hydrodynamic size distribution of the two nanoparticles in aqueous suspensions.

Here, the K+-selective sensing particles contained ETH 5294

available, the surface of the silica nanoparticles is rich in SiOH

(chromoionophore I), potassium ionophore III (BME 44), and

groups and relatively hydrophilic. Here, we propose PEGylated

cation exchanger Na TFPB. At pH 7, the particles showed

organosilica and PDMS nanospheres as the two categories of

sensing component chromoionophore in the red channel. The results indicated that the sensing components distributed quite

+

from the micromolar to

host nanomaterials for the ISOs components. Compared with

milimolar concentration range with high selectivity to other

previously reported self-assembled nanoparticles containing

highly sensitive response to K 3

+

common ions (ca. 3.2 ×10 times more selective for K over

F127 or PVC-DOS, cross-linked nanosensors are more

Na+). Similarly, the Li+-selective sensing particles contained

robust.23,35

ETH 5418 (chromoionophore VII), lithium ionophore VI, and

Organosilica nanoparticles are promising host candidates

Na TFPB. The microsensors were ca. 200 times more selective

considering the lipophilic nature of these compounds and the

to Li+ over Na+, which is in agreement with previous report on

hydrophobic side chains in the organosilica structure. To prepare

Note that the

the PEGylated organosilica nano-ISOs, we adopted a slightly

selectivity coefficients were obtained by comparing the overall

modified Stöber process.36 As shown in Scheme 1a, initially,

equilibrium constants in the theoretical optode response

several organo-silicate with different R2 substitution were

function (Eqn. S2), which was used in this work to fit the

screened to construct the core of the nanoparticles. All particles

experimental data.

were formed in ethanol-water mixture containing ammonia as

the selectivity of this lithium ionophore.

34

catalyst. Upon dispersion in aqueous solutions, however, PEGylated organosilica and PDMS nanoparticles for

nanoparticles containing the benzyl or octyl groups exhibited

ISOs. Although some silica nanoparticles are commercially

too much propensity to aggregation while particles containing octadecyl substitution flocculated easily. Therefore, only the nanoparticles containing propyl groups were selected for further modifications. In order to reduce particle aggregation, poly (ethylene oxide) groups were chemically attached onto the particles

containing

propyl

groups

by

introducing

methoxy-PEG-silane, forming a core/shell structure. Although simple surfactants such as PEG or Triton X were known to stabilize the particle dispersion, a chemical PEGylation on the particle surface could be more robust against various changes in

Scheme 1. (a) Graphical illustration of the layer by layer preparation of the PEGylated organosilica nanoparticles. (b) Steps for the fabrication of the cross-linked PDMS nanoparticles.

the particle’s environment. Figure 2 shows the TEM imaging of the PEGylated organosilica nanoparticles with diameter around 50 nm. The drying and vacuum for the TEM imaging could cause a certain degree of morphology change. Therefore,


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dynamic light scattering (DLS) provides a more in situ reflection on the nanoparticles in aqueous solutions. The hydrodynamic diameter determined from DLS was around 250 nm with a polydispersity index (PDI) of ca. 0.26, indicating a certain degree of particle aggregation in aqueous solution. Ultrasonication was found to assist the particle dispersion. On the other hand, Sylgard 184 is a well-known reagent often used to make PDMS microfluidics. As shown in Scheme 1b, the hydrophobic components of Sylgard 184 and the curing agent were used to form a nanoemulsion with F127 as the surfactant. The nanoemulsion was then brought to 65 °C to initiate the crosslinking. Figure 1b shows the TEM imaging of the cross-linked PDMS nanoparticles. The average diameter of the PDMS nanoparticles from TEM was slightly larger (ca. 70 nm) than the PEGylated organosilica ones. However, in aqueous

Figure 4. Selectivity patterns of the K+-selective (a) and the Li+ -selective (b) PDMS nano-ISOs. the sensing process of the nano-ISOs can be expressed in Eqn. 1, where Ind is the neutral chromoionophore, L the free ionophore, Mn+ the analyte ion, LMn+ the ion-carrier complex and HInd+ the protonated chromoionophore, x the complex stoichiometry, np and aq designate the nanoparticle and the aqueous phase, respectively. K1

suspensions, the PDMS nanoparticles showed a much smaller

nHIndnp+ + xL + Maqn+

hydrodynamic size (ca. 105 nm on average) and a very small

The overall equilibrium constant K1 is expressed in Eqn. 2.

PDI of ca. 0.07. This result indicates that the PDMS

The two equations are formally the same as in classical optode

nanoparticles were well dispersed in water with much less

theory based on thin polymeric films.37 However, it is worth to

aggregation, probably a consequence from the use of very

notice that K1

effective surfactant F127.

chromoionophores located on the surface of the nanospheres

here is

nIndnp + Lx Mnpn+ + nHaq+ (1)

an

average constant. Indeed,

can exhibit very different basicity compared with those in the Nano-ISOs optical responses. After the incorporation of

interior of the nanoparticles, and so could the affinity of the

the sensing components (see experimental section for details),

ionophores to the analytes vary in similar ways. Previous study of the ion-carrier stability constants and chromoionophore basicity in the nanospheres indeed pointed to such spatial distribution dependence.38,39

K1 =

[ Ind ]n [ Lx M npn + ][ H aq+ ]n [ HInd + ]n [ L]x [ M aqn + ]

(2)

Figure 3 shows the optical response of the PEGylated organosilica nano-ISOs to various ions. K+- and Li+-selective systems were studied as models. Note that the increasing absorption background towards 400 nm was a consequence of the light scattering from the particles. From the selectivity patterns, both the two systems remained highly selective to the primary ions. The logarithm selectivity coefficient of the K+-selective PEGylated organosilica nano-ISOs for K+ over Na+ was determined to be ca. -3.6, while the value of the Li+-selective ones for Li+ over K+ was ca. -1.5. Overall, the

Figure 3. Selectivity patterns of the PEGylated K+ nano-ISOs (a) and the Li+ nano-ISOs (c), and the absorption spectra of the nano-ISOs in various concentrations of K+ (b) and Li+ (d). Ion concentration are indicated on the x-axis in (a) and (c). Arrows indicate the direction for increasing ion concentrations. Insets: pictures of the nanosensors at various indicated K+ and Li+ concentrations (logarithmic).

selectivity patterns of the organosilica based systems were close to the values in the silica micro-ISOs. For samples containing a high sodium level (0.1 M or above), the detection limit of the sensor at neutral pH could be compromised. However, it was quite a different case for the PDMS-based nano-ISOs. The K+ response and the selectivity



Figure 5. Selectivity patterns of the three particles containing only ion exchanger and chromoionophore.

Page 6 of 9

Figure 6. Calibrations curves of the K+-selective PDMS nano-ISOs where the nanospheres were fabricated with different cross-linking degree. (a): 20 : 1 and (b): 5 : 1 ( PDMS elastomer base : curing agent, w/w) values (log10β) were determined for the ionophores in the three

pattern for the PDMS-based nano-ISOs were shown in Figure

matrices and summarized in Table 1 with errors estimated from

4a. The nanoparticles contained the same sensing ingredients as

curve fitting. The affinity of the lithium ionophore in the PDMS

in the PEGylated organosilica nano-ISOs, i.e., ETH 5294, BME

nano-ISOs (only ca. 102.3) was apparently too small.

44, and Na TFPB, no plasticizer used. The logarithm selectivity coefficient for K+ over Na+ in this case was only ca. -2.8.

log10 β = log10

n+ [ Lx M np ]

[ M npn + ][ L]x

=log10 K1 − log10 K 2 + pK a1 − pK a 2 (5)

Moreover, as shown in Figure 4b, when the ETH 2439, lithium ionophore VI, and Na TFPB were used on the PDMS

Table 1. Logarithmic ion-carrier stability constants (log10β) in

nanoparticles, the selectivity for Li+ completely disappeared.

the three types of silicon-based particles

The unexpected selectivity breakdown

led

us

to

investigate the reason behind. According to the ion-exchange theory, to ensure the selectivity of the ionophore, the ion-carrier complex formation constant has to be sufficiently high so that

K+ ionophore III +

Li ionophore VI

SiO2

Organosilica

PDMS

micro-ISOs

nano-ISOs

nano-ISOs

9.3±0.1

7.8±0.1

4.3±0.1

5.1±0.1

3.9±0.2

2.3±0.1

the free ion-exchange process governed by the Hofmeister series cannot compete with the ion extraction by the ionophores.

For the PDMS nano-ISOs, the degree of cross-linking

Therefore, we evaluated the optical response of the nano-ISOs

appeared to have significant effect on the sensor response. The

and the silica micro-ISOs without ionophores to different

cross-linking degree was adjusted by changing the ratio of the

concentrations of ions (only chromoionophore II and the

PDMS base components in Sylgard 184 to the curing agent

ion-exchanger Na TFPB were incorporated into the particles).

components (from 20 :1 to 5: 1). As shown in Figure 6, upon

Figure 5 shows the selectivity patterns for the three cases.

increasing the cross-linking degree, the K+ response went from a

+

Clearly, the ion-exchange between H and the evaluated cations +

sigmoidal curve to a much linear calibration line. This linear

started to occur at much lower concentrations (especially for K

calibration was not due to slow response of the nanosensors. As

and Na+) for the PDMS-based nano-ISOs. The results indicated

shown in Figure S2, the signal took only ca. 15s to stabilize

that the effective basicity of ETH 2439 in the PDMS

after changing the sample concentration. The linear calibration

nanoparticles was indeed much lower than those in the

was previously observed in our polystyrene microbeads

PEGylated

modified optodes where the sensing components were confined

organosilica

microspheres. + np

nHInd + M

Here,

n+ aq

K2

the

nanoparticles

and

ion-exchange

process

n+ np

nIndnp + M + nH

+ aq

the

silica can

be

to the surface region due to slow diffusion coefficients in polystyrene.29 High cross-linking of PDMS should also result in

(3)

expressed in Eqn. 3 while the overall equilibrium constant K2 is expressed in Eqn. 4. The ratio between K1 and K2 provides the ion-carrier complex formation constants (i.e., the stability

slower diffusion and thus cause similar response as in the polystyrene case. A linear calibration curve, nonetheless, is much simpler for the final users.

constant β, Eqn. 5, with pKa1 and pKa2 corresponding to the

K2 =

[ Ind ]n [ M npn + ][ H aq+ ]n [ HInd + ]n [ M aqn + ]

Nanosensor

(4)

loading

into

cells.

The

PEGylated

organosilica and PDMS nano-ISOs were applied to HeLa cells

chromoionophores in Eqn. 1 and Eqn. 3, respectively).40 The

incubating in DMEM cell culture. As shown in Figure 8, the nano-ISOs could enter the cells quite easily. Strong red


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pH and ions on the microspheres was demonstrated. The nano-ISOs were readily delivered into HeLa cells. The study on these miniaturized ISOs lays the foundation for further research and applications in related fields. At the same time, the selectivity difference could inspire more fundamental research concerning the underlying principles and how to make fine tuning of these parameters.

ASSOCIATED CONTENT Supporting Information Additional information as noted in the text include: chemical structures of the PEGylated organosilicate and information on

Figure 7. HeLa cells incubated with the K+-selective PEGylated organosilica (B, E, H) and the PDMS (C, F, I) nano-ISOs. A, D, and G are controls without addition of nanosensors. A, B, C: phase contrast mode, D, E, F: fluorescence mode, G, H, I: overlay. Scale bar: 50 μm.

the components of Sylgard 184 and the curing agents, supplementary figures of the PDMS nano-ISOs response, comparison between nanosensors. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

fluorescence from the chromoionophores was observed after 30

Corresponding Author

minutes of incubation. After incubating the cells with the

* Email: [email protected]

nanosensors for 24 hours, we observed little influence in the

ORCID

growth of the cells (see Figure S3 in the supporting information).

Xiaojiang Xie: 0000-0003-2629-8362

With the current ionophore/chromoionophore combination, the

Notes

potassium

# The authors contributed equally.

sensing

range

is

lower

than

the

desired

concentrations for the intracellular K+ concentration (ca. 140

The authors declare no competing financial interest.

+

mM). H chromoionophores with higher basicity or potassium ionophores with lower affinity to K+ could help adjust the response

window.

For

instance,

the

combination

of

ACKNOWLEDGEMENTS The authors thank the start fund of SUSTC and the Thousand Talents Program of China for financial support.

chromoionophore III, Na TFPB and potassium ionophore III with the organosilica nanospheres could be promising. Further investigation on the quantification of ions and related issues such as possible sequestration and toxicity are continued in the lab.

In summary, this work presented an ionophore-based optical nanosensing platform on PDMS, PEGylated organosilica and

SiO2

(1)

Xie, X.; Zhai, J.; Bakker, E. Anal. Chem. 2014, 86, 2853.

(2)

Xie, L.; Qin, Y.; Chen, H.-Y. Anal. Chem. 2013, 85, 2617.

(3)

Morf, W. E.; Seiler, K.; Rusterholz, B.; Simon, W. Anal.

Chem. 1990, 62, 738.

CONCLUSIONS

nanoparticles

REFERENCES

microspheres.

The

PEGylated

(4)

Lerchi, M.; Bakker, E.; Rusterholz, B.; Simon, W. Anal.

Chem. 1992, 64, 1534. (5)

Lerchi, M.; Reitter, E.; Simon, W.; Pretsch, E.;

organosilica nano-ISOs and SiO2 micro-ISOs resulted in better selectivity than the PDMS nano-ISOs owing to the higher ion-carrier complex formation constants. On the other hand, the cross-linking degree of the PDMS nano-ISOs could be

Chowdhury, D. A.; Kamata, S. Anal. Chem. 1994, 66, 1713. (6)

Xie, X.; Li, X.; Ge, Y.; Qin, Y.; Chen, H.-Y. Sens.

Actuators B-Chem. 2010, 151, 71.

increased to give a linear sensor calibration. Both the Badr, I. H. A.; Meyerhoff, M. E. J. Am. Chem. Soc. 2005,

organosilica nanoparticles and the SiO2 microspheres could be

(7)

readily modified through the Stöber process. A dual sensing of

127, 5318.



(8) Badr, I. H. A.; Johnson, R. D.; Diaz, M.; Hawthorne, M. F.;

(25) Lee, C. H.; Folz, J.; Zhang, W.; Jo, J.; Tan, J. W. Y.; Wang,

Bachas, L. G. Anal. Chem. 2000, 72, 4249.

X.; Kopelman, R. Anal. Chem. 2017, 89, 7943.

(9) Huber, C.; Klimant, I.; Krause, C.; Werner, T.; Wolfbeis, O.

(26) Yang, C.; Qin, Y.; Jiang, D.; Chen, H.-y. ACS Appl. Mater.

S. Anal. Chim. Acta 2001, 449, 81.

Interfaces 2016, 8, 19892.

(10) Dai, S.; Ye, Q.; Wang, E.; Meyerhoff, M. E. Anal. Chem.

(27) Telting-Diaz, M.; Bakker, E. Anal. Chem. 2002, 74, 5251.

2000, 72, 3142.

(28) Waltersa, J. D.; Hall, E. A. H. Analyst 2011, 136, 4718.

(11) Wang, X.; Mahoney, M.; Meyerhoff, M. E. Anal. Chem.

(29) Xie, X.; Crespo, G. A.; Zhai, J.; Szilágyi, I.; Bakker, E.

2017, 89, 12334.

Chem. Commun. 2014, 50, 4592.

(12) Dubach, J. M.; Das, S.; Rosenzweig, A.; Clark, H. A. Proc.

(30) Dubach, J. M.; Harjes, D. I.; Clark, H. A. J Am Chem Soc

Natl. Acad. Sci. 2009, 106, 16145.

2007, 129, 8418.

(13) Xie, X.; Zhai, J.; Bakker, E. J. Am. Chem. Soc. 2014, 136,

(31) Kisiel, A.; Klucinska, K.; Glebicka, Z.; Gniadek, M.;

16465.

Maksymiuk, K.; Michalska, A. Analyst 2014, 139, 2515.

(14) Rong, G.; Corrie, S. R.; Clark, H. A. ACS Sens. 2017, 2,

(32) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C.

327.

G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171.

(15) Dubach, J. M.; Harjes, D. I.; Clark, H. A. J. Am. Chem.

(33) Ciriminna, R.; Sciortino, M.; Alonzo, G.; Schrijver, A. d.;

Soc. 2007, 129, 8418.

Pagliaro, M. Chem. Rev. 2011, 111, 765.

(16) Xie, X.; Bakker, E. Anal. Bioanal. Chem. 2015, 407,

(34) Zhai, J.; Xie, X.; Cherubini, T.; Bakker, E. ACS Sens.

3899.

2017, 2, 606.

(17) Brasuel, M.; Kopelman, R.; Miller, T. J.; Tjalkens, R.;

(35) Xie, X.; Szilagyi, I.; Zhai, J.; Wang, L.; Bakker, E. ACS

Philbert, M. A. Anal. Chem. 2001, 73, 2221.

Sens. 2016, 1, 516.

(18) Balaconis, M. K.; Clark, H. A. Anal. Chem. 2012, 84,

(36) Chen, Y.; Meng, Q.; Wu, M.; Wang, S.; Xu, P.; Chen, H.;

5787.

Li, Y.; Zhang, L.; Wang, L.; Shi, J. J. Am. Chem. Soc. 2014, 136,

(19) Klucinska, K.; Stelmach, E.; Kisiel, A.; Maksymiuk, K.;

16326.

Michalska, A. Anal. Chem. 2016, 88, 5644.

(37) Bakker, E.; Bühlmann, P.; Pretsch, E. Chem. Rev. 1997, 97,

(20) Ruedas-Rama, M. J.; Wang, X.; Hall, E. A. H. Chem.

3083.

Commun. 2007, 1544.

(38) Xie, X.; Bakker, E. Anal. Chem. 2015, 87, 11587.

(21) Ruedas-Rama, M. J.; Hall, E. A. H. Analyst 2006, 131,

(39) Xie, X.; Zhai, J.; Jarolímová, Z. k.; Bakker, E. Anal. Chem.

1282.

2015, 88, 3015.

(22) Cash, K. J.; Li, C.; Xia, J.; Wang, L. V.; Clark, H. A. ACS

(40) Qin, Y.; Bakker, E. Talanta 2002, 58, 909.

Nano 2015, 9, 1692. (23) Xie, X.; Mistlberger, G.; Bakker, E. Anal. Chem. 2013, 85, 9932. (24) Du, X.; Xie, X. ACS Sens. 2017, 2, 1410.


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