Porous sol-gel material comprising at least one probe molecule chosen from the group consisting of croconic acid, p -dimethylaminobenzaldehyde (DMABA), p-dimethylaminocinnamaldehyde (DMACA), p -methoxybenzaldehyde and 4-methoxy-1-naphtaldehyde characterized in that said sol-gel material has an intrinsic pH lower than 1 and notably an intrinsic pH lower than 0 and characterized in that said sol-gel material is essentially prepared from: a mixture of: - at least one alkoxysilane of formula Si(OR 1) 4 or R 2 Si(OR 3) 3 wherein R 1, R 2 and R 3, identical or different, are an alkyl group with 1 to 6 carbon atoms, and - at least one alkoxysilane of formula R 4 Si(OR 5) 3 or (R 6 O) 3 Si-CH 2- CH 2 -Si(OR 7) 3 wherein R 4 is a substituted alkyl group with 1 to 6 carbon atoms, and R 5, R 6 and R 7, identical or different, are an alkyl group with 1 to 6 carbon atoms; the molar ratio of alkoxysilane(s) of formula Si (OR 1) 4 or R 2 Si(OR 3) 3 / alkoxysilane (s) of formula R 4 Si (OR 5) 3 or (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 being between 1:0.02 and 1:0.80; or a single alkoxysilane of formula (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 wherein R 6 and R 7, identical or different, are an alkyl group with 1 to 6 carbon atoms; or a mixture of: - at least one alkoxysilane of formula (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 wherein R 6 and R 7, identical or different, are an alkyl group with 1 to 6 carbon atoms, and - at least one alkoxysilane of formula Si(OR 1) 4, R 2 Si(OR 3) 3 or R 4 Si(OR 5) 3 wherein R 1, R 2 and R 3, identical or different, are an alkyl group with 1 to 6 carbon atoms, R 4 is a substituted alkyl group with 1 to 6 carbon atoms, and R 5 is an alkyl group with 1 to 6 carbon atoms; the molar ratio of alkoxysilane(s) of formula (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 / alkoxysilane (s) of formula Si(OR 1) 4, R 2 Si(OR 3) 3 or R 4 Si(OR 5) 3 being between 1:1,25 and 1:50; wherein R 4 is an alkyl group with 1 to 6 carbon atoms substituted by one or more elements chosen from the group consisting of a halogen, a -NH 2 radical, a -NHR 8 radical where R 8 is an alkyl group with 1 to 6 carbon atoms, a -COOH radical, a -COOR 9 radical where R 9 is an alkyl group with 1 to 6 carbon atoms." a mixture of: - at least one alkoxysilane of formula Si(OR 1) 4 or R 2 Si(OR 3) 3 wherein R 1, R 2 and R 3, identical or different, are an alkyl group with 1 to 6 carbon atoms, and - at least one alkoxysilane of formula R 4 Si(OR 5) 3 or (R 6 O) 3 Si-CH 2- CH 2 -Si(OR 7) 3 wherein R 4 is a substituted alkyl group with 1 to 6 carbon atoms, and R 5, R 6 and R 7, identical or different, are an alkyl group with 1 to 6 carbon atoms; the molar ratio of alkoxysilane(s) of formula Si (OR 1) 4 or R 2 Si(OR 3) 3 / alkoxysilane (s) of formula R 4 Si (OR 5) 3 or (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 being between 1:0.02 and 1:0.80; or - at least one alkoxysilane of formula Si(OR 1) 4 or R 2 Si(OR 3) 3 wherein R 1, R 2 and R 3, identical or different, are an alkyl group with 1 to 6 carbon atoms, and - at least one alkoxysilane of formula R 4 Si(OR 5) 3 or (R 6 O) 3 Si-CH 2- CH 2 -Si(OR 7) 3 wherein R 4 is a substituted alkyl group with 1 to 6 carbon atoms, and R 5, R 6 and R 7, identical or different, are an alkyl group with 1 to 6 carbon atoms; the molar ratio of alkoxysilane(s) of formula Si (OR 1) 4 or R 2 Si(OR 3) 3 / alkoxysilane (s) of formula R 4 Si (OR 5) 3 or (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 being between 1:0.02 and 1:0.80; or a single alkoxysilane of formula (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 wherein R 6 and R 7, identical or different, are an alkyl group with 1 to 6 carbon atoms; or a mixture of: - at least one alkoxysilane of formula (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 wherein R 6 and R 7, identical or different, are an alkyl group with 1 to 6 carbon atoms, and - at least one alkoxysilane of formula Si(OR 1) 4, R 2 Si(OR 3) 3 or R 4 Si(OR 5) 3 wherein R 1, R 2 and R 3, identical or different, are an alkyl group with 1 to 6 carbon atoms, R 4 is a substituted alkyl group with 1 to 6 carbon atoms, and R 5 is an alkyl group with 1 to 6 carbon atoms; the molar ratio of alkoxysilane(s) of formula (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 / alkoxysilane (s) of formula Si(OR 1) 4, R 2 Si(OR 3) 3 or R 4 Si(OR 5) 3 being between 1:1,25 and 1:50; wherein R 4 is an alkyl group with 1 to 6 carbon atoms substituted by one or more elements chosen from the group consisting of a halogen, a -NH 2 radical, a -NHR 8 radical where R 8 is an alkyl group with 1 to 6 carbon atoms, a -COOH radical, a -COOR 9 radical where R 9 is an alkyl group with 1 to 6 carbon atoms." - at least one alkoxysilane of formula (R 6 O) 3 Si-CH 2 -CH 2 -Si(OR 7) 3 wherein R 6 and R 7, identical or different, are an alkyl group with 1 to 6 carbon atoms, and - at least one alkoxysilane of formula Si(OR 1) 4, R 2 Si(OR 3) 3 or R 4 Si(OR 5) 3 wherein R 1, R 2 and R 3, identical or different, are an alkyl group with 1 to 6 carbon atoms, R 4 is a substituted alkyl group with 1 to 6 carbon atoms, and R 5 is an alkyl group with 1 to 6 carbon atoms;