Querying DateTime field in SOQL dynamically

Export_String_Batch__c batchSettings = Export_String_Batch__c.getInstance('DEFAULTS');  //fetching record from custom setting list.
String str = ((DateTime)batchSettings.LastExecutedDate__c).format('yyyy-MM-dd\'T\'hh:mm:ss\'Z\'');
String queryStr = 'SELECT Id FROM Account WHERE CreatedDate ='+str;
List current = database.query(queryStr);