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Institute of food safety and environmental

China. E-mail: xulin@fzu.edu.cn; Fax: +86-

† Electronic supplementary informa10.1039/c3an01243c

Cite this: Analyst, 2013, 138, 5555

Received 26th June 2013Accepted 26th July 2013

DOI: 10.1039/c3an01243c

www.rsc.org/analyst

This journal is ª The Royal Society of

Polyhedral oligomeric silsesquioxane (POSS)-basedmultifunctional organic–silica hybrid monoliths†

Xucong Lin,* Na Zheng, Jinzhen Wang, Xiao Wang, Yanqiong Zhengand Zenghong Xie

A facile polyhedral oligomeric silsesquioxane (POSS)-based hybrid

monolith with multiple mechanisms was developed by an in situ

polymerization. High mechanical stability and good separation

capabilities to polar and hydrophobic analytes were successfully

achieved. An ideal versatile organic–silica hybrid monolith was pre-

sented for easy access to the efficient separation of various analytes.

Organic–silica hybrid monoliths, which possess the merits suchas wide-pH-range tolerance, high mechanical stability and goodpermeability, have been studied intensively as a new stationaryphase and attract great attention.1 Numerous silica hybridmonoliths with typical reversed-phase mode for the efficientseparation of hydrophobic and weak polar analytes,2 or with ahydrophilic interaction for highly polar and charged analytes,3

have been developed respectively. However, the extent of chro-matographic retention for highly polar analytes in hydrophobicmonoliths with reversed-phase mode was weak, and the sepa-ration capacity of polar monoliths to hydrophobic analyteswas also poor. The analytical coverage of various compoundswith different polarities on the common hybrid monoliths withsingle retention mode was somewhat limited. Seeking a func-tional hybrid monolith with multiple modes to match a wideapplication to polar and hydrophobic analytes is expected.

To date, several functional silica hybrid monoliths have beenfabricated for multiple separation modes via the sol–gel processwith a post-column modication of an imidazolium ionicliquid4 or a dipyridyl-immobilized ionic liquid.5 Multiple modesincluding hydrophilic interactions, hydrogen bonding, p–p andelectrostatic interactions could be involved in a monolithiccolumn, and a broad analytical coverage was achieved, whichshed light on the research of multiple separations with an intactsilica column. However, it was still time-consuming to prepare a

monitoring, Fuzhou University, 350108,

591-22866131; Tel: +86-591-22866131

tion (ESI) available. See DOI:

Chemistry 2013

post-column modied monolith with multiple steps of chem-ical reactions, and the precise control over the quantity of thefunctional sites was not easy to be achieved in the multiple-stepmodication.6 Developing a facile versatile silica hybridmonolith for the efficient separation of various compounds hasbecome an interesting point.

Polyhedral oligomeric silsesquioxane (POSS)-based silicahybrid monolith is an ideal choice, which possesses uniqueproperties including facile polymerization, good rigidity, andabundant functional moieties for the exible modication.7

With a simple in situ polymerization, many functionalizedPOSS-based silica hybrid monoliths have been fabricatedconveniently with good robustness, high column efficiency andsignicant reproducibility, and used for capillary liquid chro-matography or electrochromatography with hydrophilic inter-actions (HI),8 reversed-phase/ion exchange (RP-IE),9 andhydrophilic interaction/ion exchange (HI-IE),10 respectively. Itavoids the drawbacks in the post-column modication and hasbecome one of the hottest topics in the region of silica hybridmonolith.7,8 However, POSS-based silica hybrid monoliths formultiple separation mechanisms are still absent. Developing afacile POSS-based silica hybrid monolith for easy access to theversatile analysis of various compounds is attractive.

Herein, by an in situ polymerization of the monomers ofPOSS-methacryl substituted (POSS-MA) and vinylbenzyl tri-methyl-ammonium chloride (VBTA), a facile versatile POSS-based hybrid monolith has been developed for multiple sepa-ration interactions (Fig. 1). p–p conjugated interactions,hydrophilic interactions and cationic sites could be provided byVBTA monomers, and polar ester groups for hydrogen bondinginteractions and rigid silica cubes for the excellent stabilitycould be afforded by POSS-MAmonomers. In this work, with theoptimum polymer recipe, the functional POSS-based monolithwas prepared with multiple retention mechanisms. A charac-terization of the resultant monolith was studied in detail. Bymeans of multiple retention mechanisms, a good chromato-graphic property and satisfactory versatility for the efficientseparations of various analytes were well achieved.

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Fig. 1 Scheme for hybrid monolith and potential multiple interactions.

Fig. 2 Capillary liquid chromatography of phenols (I) and amides (II). (I) Mobilephase, ACN–buffer (5 mM ammonium formate, pH 5.0) ¼ 70/30 (v/v). (II) Mobilephase, ACN–buffer (5 mM ammonium formate, pH 6.5) ¼ 80/20 (v/v); pressure:100 psi, pump flow rate: 0.1 mL min�1.

Fig. 3 Capillary liquid chromatography of hydrophobic estrogens. Mobile phase,buffer (5 mM, pH 7.0), ACN : buffer ¼ 70 : 30, v/v; pressure: 100 psi, pump flowrate: 0.1 mLmin�1. Solutes: 1, 4-tert-octylphenol; 2, hexestrol; 3, diethylstilbestrol;4, dienestrol; and 5, bisphenol A.

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In Tables S1, S2 and Fig. S1†, effects of polymer recipes andthe characteristics of the obtained POSS-based monolith wereinvestigated respectively. Under optimum conditions, a homo-geneous morphology with spherical units agglomerated intolarger clusters inter-dispersed by large-pore channels wasobserved in Fig. S1.† A good permeability of 1.19 � 10�13 m2

and theoretical plates of 78 000 m�1 were achieved for capillaryliquid chromatography (cLC) (Table S2†, column B). The lineardependence of column pressure on the ow rate in differentmobile phases is also presented (Fig. S2†), which denotes anexcellent mechanical stability of the resultant POSS-basedmonolith in these solutions. Besides, a signicant reproduc-ibility was achieved with RSD% of run-to-run (n ¼ 6), day-to-day(n ¼ 5), column-to-column (n ¼ 6), and batch-to-batch (n ¼ 4)less than 2.6%, 3.1%, 3.6% and 4.6%, respectively (Table S3†).

A series of experiments were further studied to indicate thatthe retention mechanisms existed in the POSS-based monolith.A typical HILIC was observed when the ACN in themobile phaseexceeded 65% (Fig. S3†). The efficient separation of neutralpolar phenols was also achieved (Fig. 2I). In Fig. 2I, with theamount of hydroxyl groups increasing, the retention of polarphenols was enhanced. The elution of phenols was in the orderof their polarity. Hydrophilic interactions would be responsiblefor the separation of polar phenols. The selectivity parameterssuch as a(OH) and a(OH)2, which were dened as kresorcinol/kphenol and kphloroglucinol/kphenol and affected by the amount ofhydroxyl groups, were 5.9 and 24.4 respectively. Hydrophilicinteractions for the efficient separation of polar phenols wereproved in this POSS-based monolith.

In Fig. 2II, polar amides were also used to evaluate thechromatographic performance of the POSS-based monolith,and the separation with an acceptable resolution was achieved.The elution of polar amides such as acrylamide (octanol–water

5556 | Analyst, 2013, 138, 5555–5558

partition coefficient, log P ¼ �0.78), acetamide (log P ¼ �1.23)and formamide (log P ¼ �1.51), which possessed primaryamine groups, was in the order of the degree of hydrophilicproperties. Hydrophilic interactions were responsible for theseparation of these polar solutes. Interestingly, dime-thylformamide (log P ¼ �1.01) acting as a tertiary amine andmethylene bisacrylamide (log P ¼ �1.44) acting as a secondaryamine were eluted prior to acrylamide (log P ¼ �0.78). Theelution order was listed as tertiary amine, secondary amine andprimary amine. It went with the amount of H atoms on theacrylamide groups, which indicated that hydrogen bondingexisted and played an active role. Hydrophilic interaction andhydrogen bonding could be involved in the resultant POSS-based monolith.

A subsequent evaluation of p–p conjugated interactions hasbeen carried out by using hydrophobic estrogens as modelanalytes. As seen in Fig. 3, the elution of hexestrol (log P ¼ 5.6)and bisphenol A (log P ¼ 3.3) is in agreement with the degree ofhydrophilic ability. However, for 4-tert-octylphenol, hexestrol,diethylstilbestrol and dienestrol, the values of log Pwere similar(5.8, 5.6, 5.1 and 5.4, respectively), and their elution in theresultant POSS-based monolith was not in good agreement withtheir hydrophilic properties. These estrogens possessed variousefficient p–p conjugated structures, and their elution was morelikely in the order of the degree of p–p conjugation. It indicatedthat p–p conjugated interactions between estrogens and

This journal is ª The Royal Society of Chemistry 2013

Fig. 4 Effects of pH (I) and buffer concentration (II) on the retention factor (k) oforganic acids. Mobile phase: (I) ammonium formate buffer (80 mmol L�1) withvarious pH in ACN : buffer ¼ 75 : 25, v/v; (II) ammonium formate (pH 5.0) withvarious concentrations in ACN : buffer ¼ 75 : 25, v/v; pressure: 100 psi, pumpflow rate: 0.1 mL min�1.

Fig. 6 Capillary liquid chromatography of nucleobases and nucleosides. Mobilephase, ammonium formate buffer (5 mM, pH 4.5), ACN : buffer ¼ 85 : 15, v/v;pressure: 100 psi, pump flow rate: 0.1 mL min�1.

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aromatic groups of VBTAmonomers might play an effective rolein the separation with this POSS-based monolith.

Besides, the ion-exchange mode for charged analytes wasalso evaluated. Effects of buffer concentrations and pH valueson the retention factor (k) were studied. In Fig. 4, k values ofbenzoic acids decreased obviously with the augment of saltconcentration or the decrease of pH values, which indicatedthat anion exchange between acidic solutes and ammoniumgroups on the POSS-based monolith was active. In this work,with the buffer concentration increasing, ion-exchange inter-actions of anionic benzoic acids and cationic ammoniumgroups became weaker and resulted in a faster elution. In thebuffer with a lower pH, the deprotonation of benzoic acids wasrestrained and less negative charges were produced, thusresulting in a weaker ion exchange. It was in agreement with atypical ion-exchange chromatography reported previously.6,10

Therefore, the resultant POSS-based monolith possessedmultiple interactions including hydrophilic interactions,hydrogen bonding, p–p conjugation and anion exchange. Tofurther evaluate its application to various compounds, analysesof hydrophobic anthraquinones or highly polar nucleobasesand nucleosides were studied. In Fig. 5, a baseline separation ofanthraquinones was achieved with HILIC. Hydrophilic inter-action and hydrogen bonding might occur attributing to the

Fig. 5 Capillary liquid chromatography of anthraquinones. Mobile phase,ammonium formate buffer (5 mM, pH 7.0), ACN : buffer ¼ 70 : 30, v/v; pressure:100 psi, pump flow rate: 0.1 mL min�1. Solutes: 1, chrysophanol; 2, physcion; 3,aloe-emodin; and 4, emodin.

This journal is ª The Royal Society of Chemistry 2013

abundant –OH groups of anthraquinones. Hydrophobic analy-tes (chrysophanol and physcion) were eluted prior to the lesshydrophobic solutes (aloe-emodin and emodin). The –O–CH3

group in physcion (2) and –CH3 group in emodin (4) acting aselectron donors could reinforce p–p conjugation on theanthracene ring. And by means of p–p conjugation, an efficientseparation of pairs of chrysophanol (1)–physcion (2) or aloe-emodin (3)–emodin (4) was achieved. Additionally, in Fig. 6,highly polar and charged nucleobases and nucleosides wereeffectively separated in 12 min. Effects of ACN, pH and bufferconcentration were studied from Fig. S4–S6.† Nucleosidespossessing more hydrogen bond donors were eluted aer thecorresponding nucleobases. An increasing retention factor (k)could be observed with the increasing ACN content from 75% to95% or the decreasing buffer concentration from 25 mmol L�1

to 5 mmol L�1. Multiple mechanisms such as HI, hydrogenbonding, p–p conjugation and electrostatic interactions wereachieved for the efficient separation, which was in agreementwith previous reports.6 Compared with the typical amide-80 andZIC-HILIC polar columns, the satisfactory selectivity factors ofribose a(ribose) dened as k(nucleoside)/k(nucleic base) wereachieved in the resultant POSS-based monolith (Table S4†).

In summary, a facile versatile poly(POSS-MA-co-VBTA) silicahybrid monolith was developed. The merits including goodrigidity, high column efficiency and favorable versatility wereobtained. Multiple mechanisms could occur in a column, and abroad analytical coverage from highly polar and charged tohydrophobic compounds was achieved. An ideal versatileorganic–silica hybrid monolith was presented for easy access tothe efficient separation of various compounds and could bepromising to be used for a wide analysis.

This work was supported by NCFS (81001634, 21177022,21277026), R&D Projects (2011YQ150072 and 2011J01042).

Notes and references

1 (a) M. Chen, J. Zhang, Z. Zhang, B. Yuan, Q. Yu and Y. Feng,J. Chromatogr., A, 2013, 1284, 118; (b) X. Wang, Y. Zheng,C. Zhang, Y. Yang, X. Lin, G. Huang and Z. Xie,J. Chromatogr., A, 2012, 1239, 56.

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2 Z. Zhang, H. Lin, J. Ou, H. Qin, R. Wu, J. Dong and H. Zou,J. Chromatogr., A, 2012, 1228, 263.

3 (a) M. Wu, R. Wu, F. Wang, L. Ren, J. Dong, Z. Liu andH. Zou, Anal. Chem., 2009, 81, 3529; (b) H. Huang, Y. Jin,M. Xue, L. Yu, Q. Fu, Y. Ke, C. Chu and X. Liang, Chem.Commun., 2009, 6973.

4 (a) P. Zhang, J. Chen and L. Jia, J. Chromatogr., A, 2011, 1218,3459–3465; (b) H. Han, Q. Wang, X. Liu and S. Jiang,J. Chromatogr., A, 2012, 1246, 9.

5 S. Tang, L. Wang, H. Han, H. Qiu, X. Liu and S. Jiang, RSCAdv., 2013, 3, 7894.

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6 (a) J. Lin, J. Lin, X. Lin and Z. Xie, J. Chromatogr., A, 2009,1216, 7728–7731; (b) X. Lin, J. Lin, Y. Sun, Y. Li and Z. Xie,Analyst, 2013, 138, 771.

7 H. Lin, J. Ou, Z. Zhang, J. Dong and H. Zou, Chem. Commun.,2013, 49, 231.

8 X. Lin, X. Wang, T. Zhao, Y. Zheng, S. Liu and Z. Xie,J. Chromatogr., A, 2012, 1260, 174.

9 M. Dong, M. Wu, F. Wang, H. Qin, G. Han, J. Dong, R. Wu,M. Ye, Z. Liu and H. Zou, Anal. Chem., 2010, 82, 2907.

10 H. Lin, J. Ou, S. Tang, Z. Zhang, J. Dong, Z. Liu and H. Zou,J. Chromatogr., A, 2013, 1301, 131.

This journal is ª The Royal Society of Chemistry 2013